Spam CXXFLAGS with -I${DESTDIR}/usr/include/private, instead of GTEST_CXXFLAGS

This makes it easier for googletest users to leverage googletest, instead of
forcing them to plug GTEST_CXXFLAGS into CXXFLAGS manually (resulting in
unnecessary duplication).

I will be following this up with a more proper fix in src.libnames.mk, as
src.libnames.mk should be automatically adding this directory to
CFLAGS/CXXFLAGS when private libraries are referenced. Not doing so can result
in mismatches between base-provided private library's and ports-provided
library's headers.

While here, tweak the comment to clarify what the intent is behind spamming
CXXFLAGS.

MFC after: 5 weeks
MFC with: r345203
Reported by: asomers
Reviewed by: asomers
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D19731