This unbreaks building when some headers are under a non-standard path.
e.g. /usr/X11R6/include as on OpenBSD.
https://bugzilla.gnome.org/show_bug.cgi?id=755850
-I$(top_srcdir)/gst-libs \
-I$(top_builddir)/gst-libs \
$(GST_PLUGINS_BASE_CFLAGS) \
+ $(GL_CFLAGS) \
--add-include-path=`PKG_CONFIG_PATH="$(GST_PKG_CONFIG_PATH)" $(PKG_CONFIG) --variable=girdir gstreamer-@GST_API_VERSION@` \
--add-include-path=`PKG_CONFIG_PATH="$(GST_PKG_CONFIG_PATH)" $(PKG_CONFIG) --variable=girdir gstreamer-base-@GST_API_VERSION@` \
--library=libgstgl-@GST_API_VERSION@.la \