Only set Fortran arguments for Fortran compiler

CMakeLists.txt

@@ -1,4 +1,8 @@
 cmake_minimum_required(VERSION 3.14.0)
+
+# Include overwrites before setting up the project
+set(CMAKE_USER_MAKE_RULES_OVERRIDE ${CMAKE_CURRENT_SOURCE_DIR}/config/DefaultFlags.cmake)
+
 project(fortran_stdlib
         LANGUAGES Fortran
         DESCRIPTION "Community driven and agreed upon de facto standard library for Fortran"
@@ -22,31 +26,9 @@ include(${PROJECT_SOURCE_DIR}/cmake/stdlib.cmake)
 # --- CMake specific configuration and package data export
 add_subdirectory(config)
 
-# --- compiler options
-if(CMAKE_Fortran_COMPILER_ID STREQUAL GNU)
-  if(CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 9.0)
-    message(FATAL_ERROR "GCC Version 9 or newer required")
-  endif()
-  add_compile_options(-fimplicit-none)
-  add_compile_options(-ffree-line-length-132)
-  add_compile_options(-fno-range-check) # Needed for gfortran 9 and
-  # earlier for hash functions 
-  add_compile_options(-Wall)
-  add_compile_options(-Wextra)
-  add_compile_options(-Wimplicit-procedure)
-  # -pedantic-errors triggers a false positive for optional arguments of elemental functions,
-  # see test_optval and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95446
-  if(CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 11.0)
-    add_compile_options(-pedantic-errors)
-  endif()
-  add_compile_options(-std=f2018)
-elseif(CMAKE_Fortran_COMPILER_ID MATCHES "^Intel")
-  if(WIN32)
-    set(fortran_flags /stand:f18 /warn:declarations,general,usage,interfaces,unused)
-  else()
-    set(fortran_flags -stand f18 -warn declarations,general,usage,interfaces,unused)
-  endif()
-  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:${fortran_flags}>")
+# --- compiler selection
+if(CMAKE_Fortran_COMPILER_ID STREQUAL GNU AND CMAKE_Fortran_COMPILER_VERSION VERSION_LESS 9.0)
+  message(FATAL_ERROR "GCC Version 9 or newer required")
 endif()
 
 # --- compiler feature checks

config/DefaultFlags.cmake

@@ -0,0 +1,51 @@
+if(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
+  set(
+    CMAKE_Fortran_FLAGS_INIT
+    "-fimplicit-none"
+    "-ffree-line-length-132"
+    "-fno-range-check"
+  )
+  set(
+    CMAKE_Fortran_FLAGS_RELEASE_INIT
+  )
+  set(
+    CMAKE_Fortran_FLAGS_DEBUG_INIT
+    "-Wall"
+    "-Wextra"
+    "-Wimplicit-procedure"
+    "-std=f2018"
+  )
+elseif(CMAKE_Fortran_COMPILER_ID MATCHES "^Intel")
+  set(
+    CMAKE_Fortran_FLAGS_INIT
+  )
+  set(
+    CMAKE_Fortran_FLAGS_RELEASE_INIT
+  )
+  if(WIN32)
+    set(
+      CMAKE_Fortran_FLAGS_DEBUG_INIT
+      "/stand:f18"
+      "/warn:declarations,general,usage,interfaces,unused"
+    )
+  else()
+    set(
+      CMAKE_Fortran_FLAGS_DEBUG_INIT
+      "-stand f18"
+      "-warn declarations,general,usage,interfaces,unused"
+    )
+  endif()
+else()
+  set(
+    CMAKE_Fortran_FLAGS_INIT
+  )
+  set(
+    CMAKE_Fortran_FLAGS_RELEASE_INIT
+  )
+  set(
+    CMAKE_Fortran_FLAGS_DEBUG_INIT
+  )
+endif()
+string(REPLACE ";" " " CMAKE_Fortran_FLAGS_INIT "${CMAKE_Fortran_FLAGS_INIT}")
+string(REPLACE ";" " " CMAKE_Fortran_FLAGS_RELEASE_INIT "${CMAKE_Fortran_FLAGS_RELEASE_INIT}")
+string(REPLACE ";" " " CMAKE_Fortran_FLAGS_DEBUG_INIT "${CMAKE_Fortran_FLAGS_DEBUG_INIT}")