appending to cflags instead of replacing

[Marcus Calhoun-Lopez]

Greetings.

I am the current maintainer of gmp for the MacPorts project, and I have an
issue I was hoping to get help with.

The configure script has the very useful ability to set the flags variables
(cflags, cxxflags)  to  appropriate values but only if the variables are
initially empty.
There are a couple of flags we want to add (e.g. -isysroot to cflags and
cxxflags) but not at the cost of not having automatic addition of -mtune,
-march, etc. by the configure script.

Our current inelegant workaround is to add isysroot to the compiler names.
This in turn pollutes the __GMP_CC variable with the isysroot value in the
installed gmp.h file.
Even if we found a way to nondestructively add isysroot to cflags, it would
still pollute the __GMP_CFLAGS variable in the gmp.h file.

Is there a way to use isysroot (and similar flags) in the build but not
have it anywhere in the installed header files?

Thank you,
Marcus