Fortran: Add support for .fpp extensions

Compilers such as `gfortran` recognize `.fpp` extensions to mean a
Fortran source on which the preprocessor should run.

Fixes: #17869

Modules/CMakeFortranCompiler.cmake.in

@@ -31,7 +31,7 @@ if(CMAKE_COMPILER_IS_MINGW)
   set(MINGW 1)
 endif()
 set(CMAKE_Fortran_COMPILER_ID_RUN 1)
-set(CMAKE_Fortran_SOURCE_FILE_EXTENSIONS f;F;f77;F77;f90;F90;for;For;FOR;f95;F95)
+set(CMAKE_Fortran_SOURCE_FILE_EXTENSIONS f;F;fpp;FPP;f77;F77;f90;F90;for;For;FOR;f95;F95)
 set(CMAKE_Fortran_IGNORE_EXTENSIONS h;H;o;O;obj;OBJ;def;DEF;rc;RC)
 set(CMAKE_Fortran_LINKER_PREFERENCE 20)
 if(UNIX)