Makefile.libcompat: Make quoting for CC/CXX/CPP more future-proof

bmake's :Q is for quoting outside of double quotes, but here is inside
double quotes, and as a result it ends up quoting characters that don't
have a special meaning inside double quotes. On the surface this would
seem harmless, but POSIX sh has a strange behaviour (differing from
outside double quotes) that, inside double quotes, a backslash before a
character that never has a special meaning inside double quotes is
passed through. As a result, should LIB${_LIBCOMPAT}CFLAGS contain
something like -DFOO\(x\)=x, we would form "... -DFOO\\\(x\\\)=x ...",
which would be parsed as -DFOO\\(x\\)=x, since the parentheses are never
special inside double quotes (but the backslash itself is), not the
original -DFOO\(x\)=x as intended.

Instead, construct the whole string as a bmake expression and use :Q on
that, without the manual double quotes around everything. Note that the
:L modifier lets you treat an expression like a variable expansion and
apply modifiers to it, i.e. ${expr:L:...} is the same as tmp=expr
${tmp:...} (in essence, ignoring possible differences due to deferred
substitution).

Improves: 537f945fc89f ("Makefile.libcompat: Quote CFLAGS and CXXFLAGS for sub-make")