xorg-server.m4: just all cflags instead of just sdkdir

Instead of fetching just the sdkdir variable of xorg-server using pkg-config,
simply get all of the CFLAGS.  Aside from completeness, this helps builds in
sysroots as pkg-config knows what to do with --cflags but doesn't remap
arbitrary variables.

Signed-off-by: Ross Burton <ross.burton@intel.com>
1 job for m4 in 1 minute and 41 seconds (queued for 2 seconds)
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passed job #24507