Add configuration arguments to mathlib

[veylonni]

Hi !
I'm building Ibex with the CMake toolchain, and it seems I cannot set a specific compiler for building mathlib, because I need to pass an extra option (CC=<path_to_compiler) to the CONFIGURE step of ExternalProject_Add here :

ibex-lib/interval_lib_wrapper/gaol/CMakeLists.txt

Line 34 in 7a1e759

CFLAGS=${CMAKE_C_FLAGS}"

and here :

ibex-lib/interval_lib_wrapper/gaol/CMakeLists.txt

Line 46 in 7a1e759

CFLAGS=${CMAKE_C_FLAGS}

A solution would be to add ${MATHLIB_CONFIG_ARGS} to the CONFIGURE step, just like in the CONFIGURE step of gaol :

ibex-lib/interval_lib_wrapper/gaol/CMakeLists.txt

Line 129 in 7a1e759

${GAOL_CONFIG_ARGS}"