Set -Wno-error=null-pointer-subtraction for Clang

GCC doesn't support the warning and fails.

meson.build

@@ -16,7 +16,9 @@ else
 endif
 add_project_arguments('-Wno-error=deprecated-declarations', language: 'c')
 # unfortunately libapr has some of that in macros
-add_project_arguments('-Wno-error=null-pointer-subtraction', language: 'c')
+if meson.get_compiler('c').get_id() == 'clang'
+  add_project_arguments('-Wno-error=null-pointer-subtraction', language: 'c')
+endif
 
 apr = dependency('apr-1')
 apr_util = dependency('apr-util-1')