This fixes a few subtle problems with passing CFLAGS and CXXFLAGS as
environment variables for configure.
First it will now only add these variables to config.mk when they are
actually set. If they are unset then the default optimizations in the
Makefile are set. This avoids passing more than one conflicting
optimization level to the compiler.
Next all CFLAGS are added to CXXFLAGS to avoid issues with forgetting to
set both CFLAGS and CXXFLAGS. This results in the cxx compiler getting
passed several redundant optimization levels when both the CFLAGS and
CXXFLAGS environment variabls are used. Now these uses of CFLAGS in
Makefile.common are set to DEF_FLAGS. This allows adding $(DEF_FLAGS)
to the CXXFLAGS variable without adding redundant flags from CFLAGS.
v2: Update other build files.
