build: improve gbm_bo_get_stride check

There were 3 problems with previous version:
- it didn't take into account libgbm cflags (fatal error: gbm.h: No such file or directory)
- it was vulnerable to gbm.h include bugs (gbm.h:259:50: error: unknown type name 'size_t')
- it was checked even when libgbm was not detected
Fix it.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

configure.ac

@@ -362,7 +362,19 @@ AM_CONDITIONAL([UTERM_HAVE_DUMB], [test x$dumb_enabled = xyes])
 AM_CONDITIONAL([UTERM_HAVE_DRM], [test x$drm_enabled = xyes])
 AM_CONDITIONAL([KMSCON_HAVE_GLES2], [test x$gles2_enabled = xyes])
 
-AC_CHECK_DECLS([gbm_bo_get_stride], [], [], [[#include <gbm.h>]])
+if test x$have_gbm = xyes ; then
+        save_CFLAGS="$CFLAGS"
+        save_LIBS="$LIBS"
+        CFLAGS=$GBM_CFLAGS
+        LIBS=$GBM_LIBS
+        AC_CHECK_LIB([gbm],
+                     [gbm_bo_get_stride],
+                     [AC_DEFINE([HAVE_GBM_BO_GET_STRIDE],
+                                [1],
+                                [Define to 1 if your libgbm provides gbm_bo_get_stride])])
+        CFLAGS="$save_CFLAGS"
+        LIBS="$save_LIBS"
+fi
 
 #
 # xkbcommon keyboard backend

src/uterm_video_drm.c

@@ -83,7 +83,7 @@ static int init_rb(struct uterm_display *disp, struct drm_rb *rb)
 		return -EFAULT;
 	}
 
-#if defined(HAVE_DECL_GBM_BO_GET_STRIDE) && HAVE_DECL_GBM_BO_GET_STRIDE
+#ifdef HAVE_GBM_BO_GET_STRIDE
 	stride = gbm_bo_get_stride(rb->bo);
 #else
 	stride = gbm_bo_get_pitch(rb->bo);