Ensure module output directory is generated in configure stage (#521)

- subprojects using the stdlib's build interface will resolve the module
  output directory before it is created and error without this fix

Author: awvwgk

src/CMakeLists.txt

@@ -66,6 +66,12 @@ set(SRC
 add_library(${PROJECT_NAME} ${SRC})
 
 set(LIB_MOD_DIR ${CMAKE_CURRENT_BINARY_DIR}/mod_files/)
+# We need the module directory before we finish the configure stage since the
+# build interface might resolve before the module directory is generated by CMake
+if(NOT EXISTS "${LIB_MOD_DIR}")
+  make_directory("${LIB_MOD_DIR}")
+endif()
+
 set_target_properties(${PROJECT_NAME} PROPERTIES
     Fortran_MODULE_DIRECTORY ${LIB_MOD_DIR})
 target_include_directories(${PROJECT_NAME} PUBLIC