Allow specifying C compiler with environment variable

FPM_C_COMPILER

Author: LKedward

src/fpm.f90

@@ -4,7 +4,7 @@ module fpm
 use fpm_command_line, only: fpm_build_settings, fpm_new_settings, &
                       fpm_run_settings, fpm_install_settings, fpm_test_settings
 use fpm_dependency, only : new_dependency_tree
-use fpm_environment, only: run
+use fpm_environment, only: run, get_env
 use fpm_filesystem, only: is_dir, join_path, number_of_rows, list_files, exists, basename
 use fpm_model, only: fpm_model_t, srcfile_t, show_model, &
                     FPM_SCOPE_UNKNOWN, FPM_SCOPE_LIB, FPM_SCOPE_DEP, &
@@ -63,6 +63,7 @@ subroutine build_model(model, settings, package, error)
     endif
 
     call get_default_c_compiler(model%fortran_compiler, model%c_compiler)
+    model%c_compiler = get_env('FPM_C_COMPILER',model%c_compiler)
 
     if (is_unknown_compiler(model%fortran_compiler)) then
         write(*, '(*(a:,1x))') &