cmake: Set default module location from CMAKE_INSTALL_LIBDIR

Derive the location where to install platform modules from the
CMAKE_INSTALL_LIBDIR variable (instead of CMAKE_INSTALL_PREFIX),
instead of assuming that the directory is always called "lib/"
inside the prefix. This provides a saner default for distributors
which use other kinds of locations, like multilib/multiarch systems.

CMakeLists.txt

@@ -72,7 +72,7 @@ option(USE_SOUP2 "Build with libsoup2 instead of libsoup3" ON)
 set(COG_APPID "" CACHE STRING "Default GApplication unique identifier")
 set(COG_HOME_URI "" CACHE STRING "Default home URI")
 
-set(COG_MODULEDIR "${CMAKE_INSTALL_PREFIX}/lib/cog/modules"
+set(COG_MODULEDIR "${CMAKE_INSTALL_LIBDIR}/cog/modules"
     CACHE STRING "Default search path for loadable modules")
 
 if (NOT COG_APPID OR COG_APPID STREQUAL "")