Update f90/CMakeLists.txt to only use Fortran flags for Fortran files

Before, several flags were being specified for the ModEM target that were not
properly being wrapped in a CMake generator expression. Currently this is not a
problem at all, but it would prevent any C files to be included, as CMake will
try to use these flags when compiling a C file.

This will prevent that issue occurring if a C file is added in the future.

Author: MiCurry

f90/CMakeLists.txt

@@ -20,9 +20,10 @@ if (CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
     target_compile_options(
         ${MODEM_EXE}
         PRIVATE
-        -cpp -ffree-line-length-none -fallow-argument-mismatch
+        $<$<COMPILE_LANGUAGE:Fortran>:-cpp>
+        $<$<COMPILE_LANGUAGE:Fortran>:-ffree-line-length-none>
+        $<$<COMPILE_LANGUAGE:Fortran>:-fallow-argument-mismatch>
         $<$<CONFIG:RELEASE>:-O3>
-
         $<$<CONFIG:DEBUG>:-O0>
         $<$<CONFIG:DEBUG>:-g>
         $<$<CONFIG:DEBUG>:-fbacktrace>
@@ -38,8 +39,8 @@ elseif (CMAKE_Fortran_COMPILER_ID STREQUAL "Intel")
     target_compile_options(
         ${MODEM_EXE}
         PRIVATE
-        -fpp
-        -parallel
+        $<$<COMPILE_LANGUAGE:Fortran>-fpp>
+        $<$<COMPILE_LANGUAGE:Fortran>-parallel>
         $<$<CONFIG:RELEASE>:-O3>
         $<$<CONFIG:RELEASE>:-w>
         $<$<CONFIG:DEBUG>:-O0>