Fix armhf configure breakage due to missing libdrm_intel package

The buildbots discovered that recent changes break on Ubuntu 15.04's
armhf images:

  configure:16137: checking for SIMPLE_DMABUF_CLIENT
  configure:16144: $PKG_CONFIG --exists --print-errors "wayland-client libdrm libdrm_intel"
  Package libdrm_intel was not found in the pkg-config search path.
  ...
  configure:16194: error: Package requirements (wayland-client libdrm libdrm_intel) were not met:

  No package 'libdrm_intel' found

This patch was provided by Daniel Stone.  I've not tested it other than
verifying it does not cause build problems on x86_64.

Acked-by: Bryce Harrington <bryce@osg.samsung.com>

configure.ac

@@ -341,11 +341,15 @@ fi
 AC_ARG_ENABLE(simple-intel-dmabuf-client,
               AS_HELP_STRING([--disable-simple-intel-dmabuf-client],
                              [do not build the simple intel dmabuf client]),,
-              enable_simple_intel_dmabuf_client="yes")
-AM_CONDITIONAL(BUILD_SIMPLE_INTEL_DMABUF_CLIENT, test "x$enable_simple_intel_dmabuf_client" = "xyes")
-if test "x$enable_simple_intel_dmabuf_client" = "xyes"; then
-  PKG_CHECK_MODULES(SIMPLE_DMABUF_CLIENT, [wayland-client libdrm libdrm_intel])
+              enable_simple_intel_dmabuf_client="auto")
+if ! test "x$enable_simple_intel_dmabuf_client" = "xno"; then
+  PKG_CHECK_MODULES(SIMPLE_DMABUF_CLIENT, [wayland-client libdrm libdrm_intel],
+		    have_simple_dmabuf_client=yes, have_simple_dmabuf_client=no)
+  if test "x$have_simple_dmabuf_client" = "xno" -a "x$enable_simple_intel_dmabuf_client" = "xyes"; then
+    AC_MSG_ERROR([Intel dmabuf client explicitly enabled, but libdrm_intel couldn't be found])
+  fi
 fi
+AM_CONDITIONAL(BUILD_SIMPLE_INTEL_DMABUF_CLIENT, test "x$enable_simple_intel_dmabuf_client" = "xyes")
 
 AC_ARG_ENABLE(clients, [  --enable-clients],, enable_clients=yes)
 AM_CONDITIONAL(BUILD_CLIENTS, test x$enable_clients = xyes)