build: standardise the layout for the CMake project

Custom local modules are expected to reside in `cmake/modules`. Relocate
them and update the top-level CMakeLists.

CMakeLists.txt

@@ -16,8 +16,10 @@ set(PROJECT_VERSION_PATCH 0)
 set(PROJECT_VERSION_GFM 13)
 set(PROJECT_VERSION ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${PROJECT_VERSION_PATCH}.gfm.${PROJECT_VERSION_GFM})
 
-include("FindAsan.cmake")
-include("CheckFileOffsetBits.cmake")
+list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules)
+
+include(FindAsan)
+include(CheckFileOffsetBits)
 
 if("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_BINARY_DIR}")
     message(FATAL_ERROR "Do not build in-source.\nPlease remove CMakeCache.txt and the CMakeFiles/ directory.\nThen: mkdir build ; cd build ; cmake .. ; make")