Repurpose C_SWITCH_X_SYSTEM as GNUSTEP_CFLAGS

Repurpose C_SWITCH_X_SYSTEM as GNUSTEP_CFLAGS
This is the only thing left that uses it.

* configure.in (GNUSTEP_CFLAGS): Rename from C_SWITCH_X_SYSTEM.

* src/Makefile.in (GNUSTEP_CFLAGS): Rename from C_SWITCH_X_SYSTEM.

* lwlib/Makefile.in (C_SWITCH_X_SYSTEM): Remove.
(ALL_CFLAGS): Remove C_SWITCH_X_SYSTEM.

* oldXMenu/Makefile.in (C_SWITCH_X_SYSTEM): Remove.
(ALL_CFLAGS): Remove C_SWITCH_X_SYSTEM.

* msdos/sedlibmk.inp, msdos/sed1v2.inp:
GNUSTEP_CFLAGS replaces C_SWITCH_X_SYSTEM.