When trying to build glpk-4.61 on Mac OS X with
./configure --with-gmp
I get the following error message:
checking gmp.h usability... no
checking gmp.h presence... no
checking for gmp.h... no
configure: error: gmp.h header not found
There were no issues installing GMP, and the header file gmp.h is present in /usr/local/include as expected.
config.log told me no more than I already knew, that gcc can't find gmp.h (when trying to preprocess conftest.c). However, one can check which include paths are used by the c preprocessor with cpp -v, and it turned out that Xcode had made a mess of them. The solution was to run xcode-select --install, which added the usual include paths back.