CMake: Allow to override CDEFS

Useful cross compilation modes without an emulator that can run try/compile tests
for example osx/arm64: conda-forge/scalapack-feedstock#31
cc @traversaro

Author: jschueller

CMakeLists.txt

@@ -123,7 +123,9 @@ set(PKG_CONFIG_DIR ${libdir}/pkgconfig)
 include(FortranMangling)
 COMPILE(install_COMPILED)
 
-FORTRAN_MANGLING(CDEFS)
+if (NOT DEFINED CDEFS)
+  FORTRAN_MANGLING(CDEFS)
+endif ()
 #MESSAGE(STATUS "Setting CDEFS = ${CDEFS}")
 #set(CDEFS ${CDEFS} CACHE STRING "Fortran Mangling" FORCE)
 MESSAGE(STATUS "=========")