Remove compiler ID from include directrory

It adds the compiler ID to the include directory: include/mctc-lib/GNU-11.3.0/

This is very unconventional and unnecessary.

With this patch it now installs headers into include/mctc-lib which
aligns with the way how most projects install their headers.

config/CMakeLists.txt

@@ -19,7 +19,7 @@ option(WITH_JSON "Enable support for JSON parsing via json-fortran" FALSE)
 
 set(
   "${PROJECT_NAME}-module-dir"
-  "${PROJECT_NAME}/${CMAKE_Fortran_COMPILER_ID}-${CMAKE_Fortran_COMPILER_VERSION}"
+  "${PROJECT_NAME}"
   CACHE STRING
   "Subdirectory to install generated module files to"
 )