Bug Description

When using the openmp = "*" dependency in fpm.toml, fpm applies OpenMP compiler flags (like -fopenmp) to all source files, including C files. This causes compilation failures when the default C compiler is clang, which doesn't support -fopenmp.

Reproduction Steps

1. Create a project with both Fortran and C source files
2. Add openmp = "*" to dependencies in fpm.toml
3. Run fpm build on macOS (where clang is the default C compiler)

Expected Behavior

OpenMP flags should only be applied to Fortran files when using OpenMP dependency, or there should be a way to specify different compilers for C and Fortran files.

Actual Behavior

Build fails with:

clang: error: unsupported option '-fopenmp'

Environment

• OS: macOS
• fpm version: latest
• Default C compiler: clang (doesn't support -fopenmp)
• Default Fortran compiler: gfortran (supports -fopenmp)

Proposed Solutions

1. Only apply OpenMP flags to Fortran source files
2. Allow separate compiler configuration for C files (e.g., [build.c] section)
3. Provide environment variable to override C compiler (e.g., FPM_CC)

This affects mixed-language projects that want to use OpenMP in Fortran while having simple C utility functions.