Update CMakeLists handling of .mod files

CMakeLists.txt

@@ -2,11 +2,6 @@ cmake_minimum_required(VERSION 3.14.0)
 project(stdlib Fortran)
 enable_testing()
 
-# this avoids stdlib and projects using stdlib from having to introspect stdlib's directory structure
-# FIXME: this eventually needs to be handled more precisely, as this spills all .mod/.smod into one directory
-#  and thereby can clash if module/submodule names are the same in different parts of library
-set(CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_BINARY_DIR})
-
 # --- compiler options
 if(CMAKE_Fortran_COMPILER_ID STREQUAL GNU)
   add_compile_options(-fimplicit-none)

src/CMakeLists.txt

@@ -9,6 +9,14 @@ set(SRC
 
 add_library(fortran_stdlib ${SRC})
 
+set(LIB_MOD_DIR ${CMAKE_CURRENT_BINARY_DIR}/mod_files/)
+set_target_properties(fortran_stdlib PROPERTIES
+    Fortran_MODULE_DIRECTORY ${LIB_MOD_DIR})
+target_include_directories(fortran_stdlib PUBLIC
+    $<BUILD_INTERFACE:${LIB_MOD_DIR}>
+    $<INSTALL_INTERFACE:include>
+)
+
 if(f18errorstop)
   target_sources(fortran_stdlib PRIVATE f18estop.f90)
 else()
@@ -22,3 +30,4 @@ install(TARGETS fortran_stdlib
         ARCHIVE DESTINATION lib
         LIBRARY DESTINATION lib
     )
+install(DIRECTORY ${LIB_MOD_DIR} DESTINATION include)