fix: Use appropriate compilers for linking pure C/C++ programs

- Fix check_c_source_runs() to use C compiler (icx) instead of Fortran compiler (ifx) for linking
- Fix check_cxx_source_runs() to use C++ compiler (icpx) instead of Fortran compiler for linking
- Resolves "multiple definition of main" error with Intel Fortran compiler
- Intel Fortran automatically links for_main.o which conflicts with C main() functions
- This enables C/C++ compiler capability testing (e.g., OpenMP flag detection) to work correctly

The previous approach of using Fortran compiler for linking pure C/C++ programs
caused linker conflicts because Intel Fortran includes its own main entry point.
Using the appropriate language compiler for pure language programs is the correct approach.

Fixes test failures in check-c-source-runs and check-cxx-source-runs.
Related to OpenMP metapackage C/C++ compiler capability detection.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: krystophny

src/fpm_compiler.F90

@@ -1859,8 +1859,9 @@ logical function check_c_source_runs(self, input, compile_flags, link_flags) res
     call self%compile_c(source,object,flags,logf,stat,dry_run=.false.)
     if (stat/=0) return
 
-    !> Link  
-    call self%link(exe,ldflags//" "//object,logf,stat)
+    !> Link using C compiler for pure C programs
+    call run(self%cc//" "//ldflags//" "//object//" -o "//exe, &
+              echo=self%echo, verbose=self%verbose, redirect=logf, exitstat=stat)
     if (stat/=0) return
 
     !> Run
@@ -1915,8 +1916,9 @@ logical function check_cxx_source_runs(self, input, compile_flags, link_flags) r
     call self%compile_cpp(source,object,flags,logf,stat,dry_run=.false.)
     if (stat/=0) return
 
-    !> Link  
-    call self%link(exe,ldflags//" "//object,logf,stat)
+    !> Link using C++ compiler for pure C++ programs
+    call run(self%cxx//" "//ldflags//" "//object//" -o "//exe, &
+              echo=self%echo, verbose=self%verbose, redirect=logf, exitstat=stat)
     if (stat/=0) return
 
     !> Run