Use GNU sed if available

Fixes update-copyright.sh on macOS, which doesn't support the -b option.

build-scripts/update-copyright.sh

@@ -1,7 +1,15 @@
 #!/bin/sh
 
+if [ "$SED" = "" ]; then
+    if type gsed >/dev/null; then
+        SED=gsed
+    else
+        SED=sed
+    fi
+fi
+
 find . -type f \
 | grep -v \.git                                 \
 | while read file; do                           \
-    LC_ALL=C sed -b -i "s/\(.*Copyright.*\)[0-9]\{4\}\( *Sam Lantinga\)/\1`date +%Y`\2/" "$file"; \
+    LC_ALL=C $SED -b -i "s/\(.*Copyright.*\)[0-9]\{4\}\( *Sam Lantinga\)/\1`date +%Y`\2/" "$file"; \
 done