egl: add HAVE_LIBDRM define, fix EGL X11 platform

egl: add HAVE_LIBDRM define, fix EGL X11 platform

Commit a594cec broke EGL X11 backend by adding dependency between
X11 and DRM backends requiring HAVE_EGL_PLATFORM_DRM defined for X11.

This patch fixes the issue by adding additional define for libdrm
detection independent of which backend is being compiled. Tested by
compiling Mesa with '--with-egl-platforms=x11' and running es2gears_x11
+ glbenchmark2.7 successfully.

v2: return true for dri2_auth if running without libdrm (Samuel)
v3: check libdrm when building EGL drm platform + AM_CFLAGS fix (Emil)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72062
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: mesa-stable@lists.freedesktop.org