check existence of headers and libs for WinCE in Makefile.ce

Macro LIBC was used since day 1 of WinCE port (commit e1caacb4fd ), but
never before defined. On Desktop Win32 Perl, LIBC is msvcrt.lib, so fix
corelibc/msvcrt to be LIBC and not part of CELIBS. Then use LIBC in a
sanity check. Sanity checks will give an error message, vs running the
C compiler and geting cryptic messages about unknown .hs and random
missing symbols.