build_proton: fix the hardcoded absolute path name in libSDL2.dylib

Without this fix, wine fails to dlopen dinput.dll.so, as it looks for
libSDL2.dylib in its original path where it was built, rather than in
the libdirs next to the wine libraries.

Ideally, we'd fix this in the libSDL build rather than postprocessing
it, but I lack the familiarity with libSDL's buildsystem with regard to
darwin.

Signed-off-by: Christopher Larson <kergoth@gmail.com>

build_proton.sh

@@ -175,10 +175,12 @@ function build_libSDL
     fi
 
     cp "$TOOLS_DIR32"/lib/libSDL2.$LIB_SUFFIX "$DST_DIR"/lib
-    $STRIP "$DST_DIR"/lib/libSDL2.dylib
+    $STRIP "$DST_DIR"/lib/libSDL2.$LIB_SUFFIX
+    install_name_tool -id @rpath/libSDL2.$LIB_SUFFIX "$DST_DIR"/lib/libSDL2.$LIB_SUFFIX
 
     cp "$TOOLS_DIR64"/lib/libSDL2.$LIB_SUFFIX "$DST_DIR"/lib64
-    $STRIP "$DST_DIR"/lib64/libSDL2.dylib
+    $STRIP "$DST_DIR"/lib64/libSDL2.$LIB_SUFFIX
+    install_name_tool -id @rpath/libSDL2.$LIB_SUFFIX "$DST_DIR"/lib64/libSDL2.$LIB_SUFFIX
 }
 
 function build_moltenvk