Move "system type" macros from src/s to configure

Move "system type" macros from src/s to configure

* configure.ac (BSD4_2, BSD_SYSTEM, USG, USG5, _AIX, CYGWIN)
(DARWIN_OS, GNU_LINUX, HPUX, IRIX6_5, SOLARIS2):
Move "system type" macros here from src/s.
(BSD_SYSTEM_AHB): New hack macro.
(AH_BOTTOM): Set BSD_SYSTEM, using BSD_SYSTEM_AHB.

* src/s/usg5-4-common.h (USG, USG5):
* src/s/template.h (USG5, USG, HPUX, BSD4_2, BSD_SYSTEM):
* src/s/sol2-6.h (SOLARIS2):
* src/s/irix6-5.h (IRIX6_5):
* src/s/hpux10-20.h (USG, USG5, HPUX):
* src/s/gnu-linux.h (USG, GNU_LINUX):
* src/s/freebsd.h (BSD_SYSTEM):
* src/s/darwin.h (BSD4_2, BSD_SYSTEM, DARWIN_OS):
* src/s/cygwin.h (CYGWIN):
* src/s/bsd-common.h (BSD_SYSTEM, BSD4_2):
* src/s/aix4-2.h (USG, USG5, _AIX): Move "system type" macros to configure.