Pass CPPFLAGS/CFLAGS/CXXFLAGS from the environment of configure to
Makefile.config.

This is implied at the bottom of the help text of configure (besides
CC/CXX/LDFLAGS, already passed to Makefile.config).

For backward compatibility, the values of CFLAGS and CXXFLAGS defaults
to empty, overriding the default values provided by autoconf (for
example, '-g -O2' when CC=gcc').

$(CPP) is not used by our makefiles. Therefore, the value of CPP is
not passed to Makefile.config, despite beeing mentioned by 'configure
--help'.