sdl2.m4: Call $PKG_CONFIG rather than hard-coding pkg-config

When cross-compiling, pkg.m4 will set the PKG_CONFIG variable
to a suitable pkg-config executable, which might be a
host-architecture-prefixed name like aarch64-linux-gnu-pkg-config. This
ensures that we will use a suitable search path that is suitable for
the host architecture (the machine we are compiling for) rather than
the build architecture (the machine we are compiling on) to find
dependency libraries like SDL. When using pkg-config as a substitute
for sdl2-config, we will similarly need to use the
host-architecture-prefixed pkg-config.

Setting the PKG_CONFIG environment variable is also the canonical way to
request that a different pkg-config implementation be used, if that
becomes necessary for some reason.

Co-authored-by: Helmut Grohne <helmut@subdivi.de>
Bug-Debian: https://bugs.debian.org/1059749
Signed-off-by: Simon McVittie <smcv@collabora.com>

sdl2.m4

@@ -9,8 +9,9 @@
 # * also look for SDL2.framework under Mac OS X
 # * removed HP/UX 9 support.
 # * updated for newer autoconf.
+# * (v3) use $PKG_CONFIG for pkg-config cross-compiling support
 
-# serial 2
+# serial 3
 
 dnl AM_PATH_SDL2([MINIMUM-VERSION, [ACTION-IF-FOUND [, ACTION-IF-NOT-FOUND]]])
 dnl Test for SDL, and define SDL_CFLAGS and SDL_LIBS
@@ -54,7 +55,7 @@ AC_ARG_VAR(SDL2_FRAMEWORK, [Path to SDL2.framework])
 
   if test "x$sdl_pc" = xyes ; then
     no_sdl=""
-    SDL2_CONFIG="pkg-config sdl2"
+    SDL2_CONFIG="$PKG_CONFIG sdl2"
   else
     as_save_PATH="$PATH"
     if test "x$prefix" != xNONE && test "$cross_compiling" != yes; then