[configure] Add some heuristics to find drm headers

On my system, some headers seem to be stuffed in /usr/include/libdrm
and other headers seem to be stuffed in /usr/include/drm .

I think the ultimate upstream goal is for everything to be in
/usr/include/libdrm but my system seems to be in some transition state.

My pkgconfig files only point me to one of the directories, so add some
heuristics to find the other one.

configure.ac

@@ -57,6 +57,45 @@ AC_SUBST(GTK_CFLAGS)
 AC_SUBST(GTK_LIBS)
 
 PKG_CHECK_MODULES(DRM, [libdrm libdrm_intel libdrm_radeon libdrm_nouveau])
+
+OLD_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS $DRM_CFLAGS"
+AC_MSG_CHECKING([if i915_drm.h, radeon_drm.h and nouveau_drm.h are in include path])
+AC_COMPILE_IFELSE(AC_LANG_PROGRAM(
+[[
+  #include <stdint.h>
+  #include <stdlib.h>
+  #include "i915_drm.h"
+  #include "radeon_drm.h"
+  #include "nouveau_drm.h"
+]],[[]]),[found_drm_kernel_headers=yes],[found_drm_kernel_headers=no])
+
+if test "$found_drm_kernel_headers" = "yes"; then
+  AC_MSG_RESULT([yes])
+else
+  AC_MSG_RESULT([no])
+  AC_MSG_CHECKING([if we can find them anyway])
+
+  MORE_DRM_CFLAGS="-I`$PKG_CONFIG --variable includedir libdrm`/drm"
+  CFLAGS="$CFLAGS $MORE_DRM_CFLAGS"
+  AC_COMPILE_IFELSE(AC_LANG_PROGRAM(
+  [[
+    #include <stdint.h>
+    #include <stdlib.h>
+    #include "i915_drm.h"
+    #include "radeon_drm.h"
+    #include "nouveau_drm.h"
+  ]],[[]]),[found_drm_kernel_headers=yes],[found_drm_kernel_headers=no])
+
+  if test "$found_drm_kernel_headers" = "yes"; then
+    AC_MSG_RESULT([yes])
+    DRM_CFLAGS="$DRM_CFLAGS $MORE_DRM_CFLAGS"
+  else
+    AC_MSG_RESULT([no])
+    AC_MSG_ERROR([Could not find i915_drm.h, radeon_drm.h and/or nouveau_drm.h])
+  fi
+fi
+CFLAGS="$OLD_CFLAGS"
 AC_SUBST(DRM_CFLAGS)
 AC_SUBST(DRM_LIBS)