x11: On macOS, look for X11 install in /opt/X11 instead of /usr/X11R6.

This is where Apple installs XQuartz now (and apparently, the compatibility
symlink at /usr/X11R6 can be missing).

Fixes Bugzilla #4706.

configure

@@ -19886,10 +19886,10 @@ fi
                 # This isn't necessary for X11, but fixes GLX detection
                 if test "x$x_includes" = xNONE && \
                    test "x$x_libraries" = xNONE && \
-                   test -d /usr/X11R6/include && \
-                   test -d /usr/X11R6/lib; then
-                    x_includes="/usr/X11R6/include"
-                    x_libraries="/usr/X11R6/lib"
+                   test -d /opt/X11/include && \
+                   test -d /opt/X11/lib; then
+                    x_includes="/opt/X11/include"
+                    x_libraries="/opt/X11/lib"
                 fi
                 ;;
         esac
@@ -20587,15 +20587,16 @@ fi
 
             case "$host" in
                 *-*-darwin*)
-                    x11_lib='/usr/X11R6/lib/libX11.6.dylib'
-                    x11ext_lib='/usr/X11R6/lib/libXext.6.dylib'
-                    xcursor_lib='/usr/X11R6/lib/libXcursor.1.dylib'
-                    xinerama_lib='/usr/X11R6/lib/libXinerama.1.dylib'
-                    xinput_lib='/usr/X11R6/lib/libXi.6.dylib'
-                    xrandr_lib='/usr/X11R6/lib/libXrandr.2.dylib'
-                    xrender_lib='/usr/X11R6/lib/libXrender.1.dylib'
-                    xss_lib='/usr/X11R6/lib/libXss.1.dylib'
-                    xvidmode_lib='/usr/X11R6/lib/libXxf86vm.1.dylib'
+                    # Apple now puts this in /opt/X11
+                    x11_lib='/opt/X11/lib/libX11.6.dylib'
+                    x11ext_lib='/opt/X11/lib/libXext.6.dylib'
+                    xcursor_lib='/opt/X11/lib/libXcursor.1.dylib'
+                    xinerama_lib='/opt/X11/lib/libXinerama.1.dylib'
+                    xinput_lib='/opt/X11/lib/libXi.6.dylib'
+                    xrandr_lib='/opt/X11/lib/libXrandr.2.dylib'
+                    xrender_lib='/opt/X11/lib/libXrender.1.dylib'
+                    xss_lib='/opt/X11/lib/libXss.1.dylib'
+                    xvidmode_lib='/opt/X11/lib/libXxf86vm.1.dylib'
                     ;;
                 *-*-openbsd*)
                     x11_lib='libX11.so'

configure.ac

@@ -1610,10 +1610,10 @@ AS_HELP_STRING([--enable-video-x11], [use X11 video driver [[default=yes]]]),
                 # This isn't necessary for X11, but fixes GLX detection
                 if test "x$x_includes" = xNONE && \
                    test "x$x_libraries" = xNONE && \
-                   test -d /usr/X11R6/include && \
-                   test -d /usr/X11R6/lib; then
-                    x_includes="/usr/X11R6/include"
-                    x_libraries="/usr/X11R6/lib"
+                   test -d /opt/X11/include && \
+                   test -d /opt/X11/lib; then
+                    x_includes="/opt/X11/include"
+                    x_libraries="/opt/X11/lib"
                 fi
                 ;;
         esac
@@ -1626,15 +1626,16 @@ AS_HELP_STRING([--enable-x11-shared], [dynamically load X11 support [[default=ma
 
             case "$host" in
                 *-*-darwin*)
-                    x11_lib='/usr/X11R6/lib/libX11.6.dylib'
-                    x11ext_lib='/usr/X11R6/lib/libXext.6.dylib'
-                    xcursor_lib='/usr/X11R6/lib/libXcursor.1.dylib'
-                    xinerama_lib='/usr/X11R6/lib/libXinerama.1.dylib'
-                    xinput_lib='/usr/X11R6/lib/libXi.6.dylib'
-                    xrandr_lib='/usr/X11R6/lib/libXrandr.2.dylib'
-                    xrender_lib='/usr/X11R6/lib/libXrender.1.dylib'
-                    xss_lib='/usr/X11R6/lib/libXss.1.dylib'
-                    xvidmode_lib='/usr/X11R6/lib/libXxf86vm.1.dylib'
+                    # Apple now puts this in /opt/X11
+                    x11_lib='/opt/X11/lib/libX11.6.dylib'
+                    x11ext_lib='/opt/X11/lib/libXext.6.dylib'
+                    xcursor_lib='/opt/X11/lib/libXcursor.1.dylib'
+                    xinerama_lib='/opt/X11/lib/libXinerama.1.dylib'
+                    xinput_lib='/opt/X11/lib/libXi.6.dylib'
+                    xrandr_lib='/opt/X11/lib/libXrandr.2.dylib'
+                    xrender_lib='/opt/X11/lib/libXrender.1.dylib'
+                    xss_lib='/opt/X11/lib/libXss.1.dylib'
+                    xvidmode_lib='/opt/X11/lib/libXxf86vm.1.dylib'
                     ;;
                 *-*-openbsd*)
                     x11_lib='libX11.so'

include/SDL_config_macosx.h

@@ -159,13 +159,13 @@
 #define SDL_VIDEO_DRIVER_COCOA  1
 #define SDL_VIDEO_DRIVER_DUMMY  1
 #undef SDL_VIDEO_DRIVER_X11
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC "/usr/X11R6/lib/libX11.6.dylib"
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XEXT "/usr/X11R6/lib/libXext.6.dylib"
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XINERAMA "/usr/X11R6/lib/libXinerama.1.dylib"
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XINPUT2 "/usr/X11R6/lib/libXi.6.dylib"
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XRANDR "/usr/X11R6/lib/libXrandr.2.dylib"
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XSS "/usr/X11R6/lib/libXss.1.dylib"
-#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XVIDMODE "/usr/X11R6/lib/libXxf86vm.1.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC "/opt/X11/lib/libX11.6.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XEXT "/opt/X11/lib/libXext.6.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XINERAMA "/opt/X11/lib/libXinerama.1.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XINPUT2 "/opt/X11/lib/libXi.6.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XRANDR "/opt/X11/lib/libXrandr.2.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XSS "/opt/X11/lib/libXss.1.dylib"
+#define SDL_VIDEO_DRIVER_X11_DYNAMIC_XVIDMODE "/opt/X11/lib/libXxf86vm.1.dylib"
 #define SDL_VIDEO_DRIVER_X11_XDBE 1
 #define SDL_VIDEO_DRIVER_X11_XINERAMA 1
 #define SDL_VIDEO_DRIVER_X11_XRANDR 1

src/video/x11/SDL_x11opengl.c

@@ -36,7 +36,7 @@
 /* IRIX doesn't have a GL library versioning system */
 #define DEFAULT_OPENGL  "libGL.so"
 #elif defined(__MACOSX__)
-#define DEFAULT_OPENGL  "/usr/X11R6/lib/libGL.1.dylib"
+#define DEFAULT_OPENGL  "/opt/X11/lib/libGL.1.dylib"
 #elif defined(__QNXNTO__)
 #define DEFAULT_OPENGL  "libGL.so.3"
 #else