meson: get opengl compilation flags from OPENGL_CFLAGS

The opengl compilation flags were added to QEMU_CFLAGS.  We do not
want them to be added to all compilation commands, so export them
also via OPENGL_CFLAGS rather than via QEMU_CFLAGS.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/configure b/configure
index f2a3a11..1b1a4ce 100755
--- a/configure
+++ b/configure
@@ -7054,6 +7054,7 @@
 
 if test "$opengl" = "yes" ; then
   echo "CONFIG_OPENGL=y" >> $config_host_mak
+  echo "OPENGL_CFLAGS=$opengl_cflags" >> $config_host_mak
   echo "OPENGL_LIBS=$opengl_libs" >> $config_host_mak
   if test "$opengl_dmabuf" = "yes" ; then
     echo "CONFIG_OPENGL_DMABUF=y" >> $config_host_mak
diff --git a/meson.build b/meson.build
index 689fc22..071ef26 100644
--- a/meson.build
+++ b/meson.build
@@ -307,7 +307,8 @@
 endif
 opengl = not_found
 if 'CONFIG_OPENGL' in config_host
-  opengl = declare_dependency(link_args: config_host['OPENGL_LIBS'].split())
+  opengl = declare_dependency(compile_args: config_host['OPENGL_CFLAGS'].split(),
+                              link_args: config_host['OPENGL_LIBS'].split())
 else
 endif
 gtk = not_found