Makefile.libcompat: Quote CFLAGS and CXXFLAGS for sub-make

Currently none of the words in these require quoting, but a future
commit will add words that do, thus we should make sure to quote each
word so the shell doesn't mangle them before calling the sub-make.

(Note that :@var@expr@ is the bmake syntax for map, replacing each word
with expr's evaluation, with var containing the input word)

Reviewed by: emaste, brooks, jhb
Differential Revision: https://reviews.freebsd.org/D40921