Avoid conflicts with Solaris <sys/regset.h> defines that clash with our names

Avoid conflicts with Solaris <sys/regset.h> defines that clash with our names

When building on Solaris with _XOPEN_SOURCE set to a recent XPG release,
<stdlib.h> and other core headers start including <sys/regset.h>, which
has a bunch of unfortunately named macros such as "CS", "ES", etc. for
x86 & x64 registers which clash with existing variable & struct member
names in Xorg - so #undef these so they don't interfere with our use.

(Yes, have filed a bug against the system headers for exposing these,
 but this solves the problem for building on existing releases.)

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>