Fix CMake install command
Headers should be installed to the correct includes directory. `"."`
installs the installation prefix root, and `OPTIONAL EXCLUDE_FROM_ALL`
causes the headers to just not be installed.
@@ -10,7 +10,8 @@ option(CLAP_BUILD_TESTS "Should CLAP build tests and the like?" OFF)
add_library(clap-core INTERFACE)
target_include_directories(clap-core INTERFACE include)
-install(DIRECTORY include DESTINATION "." OPTIONAL EXCLUDE_FROM_ALL)
+include(GNUInstallDirs)
+install(DIRECTORY "include/clap" DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
# clap-tests should always be available, to avoid build failing here and there
# because the target doesn't exists
