pixman: define pixman_have_{mmx,sse2} on 64-bit Linux

pixman: define pixman_have_{mmx,sse2} on 64-bit Linux

The refactoring of pixman removed pixman-sse2.h and pixman-mmx.h in commit
41a9a17e0308f2075bb1bd59c4411e43a67d49ec
(http://cgit.freedesktop.org/pixman/commit/?id=41a9a17e0308f2075bb1bd59c4411e43a67d49ec).
On 64-bit Linux this breaks linking of new programs as well as execution of
existing programs with the following errors:

../pixman/.libs/libpixman-1.so: undefined reference to `pixman_have_mmx'
../pixman/.libs/libpixman-1.so: undefined reference to `pixman_have_sse2'

This patch fixes the issue for me by re-introducing the definitions for these
functions. It might be preferable, though, to create proper trivial static
inline functions instead.

Signed-off-by: Magnus Kessler <Magnus.Kessler@gmx.net>