[c]: use GNUInstallDirs to help determine installation paths

In particular, this handles cases where libraries should be installed in
e.g. `/usr/lib64` rather than `/usr/lib`.

https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html

c/CMakeLists.txt

@@ -131,10 +131,11 @@ add_custom_command(
     COMMAND ${CMAKE_CTEST_COMMAND}
 )
 ############ Installation section ############
-set(include_install_dir "include")
-set(lib_install_dir "lib/")
-set(bin_install_dir "bin/")
-set(config_install_dir "lib/cmake/")
+include(GNUInstallDirs)
+set(include_install_dir "${CMAKE_INSTALL_INCLUDEDIR}")
+set(lib_install_dir "${CMAKE_INSTALL_LIBDIR}/")
+set(bin_install_dir "${CMAKE_INSTALL_BINDIR}/")
+set(config_install_dir "${CMAKE_INSTALL_LIBDIR}/cmake/")
 
 set(project_config "${CMAKE_CURRENT_BINARY_DIR}/generated/${PROJECT_NAME}Config.cmake")
 set(targets_export_name "${PROJECT_NAME}Targets")