man: rework the Makefile.am

Remove GNU make specific constructs and take into consideration that
Solaris man 7 is not the same as Linux man 7.

This commit introduces a dependency of xorg-macros 1.12 (released 4+
years ago) which is used to handle the above man section discrepancies.

Cc: Niveditha Rau <niveditha.rau@oracle.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>