cmake: support gnu installation directory convention

Support the handling of the `DESTDIR` environment variable for
environments (UNIX-based) opting for an GNU-style installation over
a CMake-style installation.

Cc: sl1pkn07 <sl1pkn07@gmail.com>
Signed-off-by: James Knight <james.d.knight@live.com>

CMakeLists.txt

@@ -55,6 +55,6 @@ endforeach()
 # legacy
 add_custom_target(install-headers
     COMMAND cmake -E copy_directory ${CMAKE_CURRENT_SOURCE_DIR}/include/spirv
-        ${CMAKE_INSTALL_PREFIX}/include/spirv)
+        $ENV{DESTDIR}${CMAKE_INSTALL_PREFIX}/include/spirv)
 
 add_subdirectory(example)