Move LIB_GCC from cpp to configure.

Move LIB_GCC from cpp to configure.

* configure.in (LIB_GCC): New output variable.

* src/Makefile.in (LIB_GCC): Set using configure, not cpp.
(GNULIB_VAR) [!ORDINARY_LINK]: Always set to $LIB_GCC.
* src/m/arm.h (LIB_GCC) [GNU_LINUX]:
* src/s/cygwin.h (LIB_GCC):
* src/s/freebsd.h (LIB_GCC):
* src/s/gnu-linux.h (LIB_GCC):
* src/s/msdos.h (LIB_GCC):
* src/s/netbsd.h (LIB_GCC):
Move to configure.

* msdos/sed1v2.inp (LIB_GCC): Edit to -Lgcc.