meson: strip build relative paths from file macros

meson.build

@@ -10,12 +10,41 @@ project('foot', 'c',
 
 is_debug_build = get_option('buildtype').startswith('debug')
 
+cc = meson.get_compiler('c')
+
 add_project_arguments(
   ['-D_GNU_SOURCE=200809L'] + (is_debug_build ? ['-D_DEBUG'] : []),
   language: 'c',
 )
 
-cc = meson.get_compiler('c')
+# Compute the relative path used by compiler invocations.
+source_root = meson.current_source_dir().split('/')
+build_root = meson.build_root().split('/')
+relative_dir_parts = []
+i = 0
+in_prefix = true
+foreach p : build_root
+  if i >= source_root.length() or not in_prefix or p != source_root[i]
+    in_prefix = false
+    relative_dir_parts += '..'
+  endif
+  i += 1
+endforeach
+i = 0
+in_prefix = true
+foreach p : source_root
+  if i >= build_root.length() or not in_prefix or build_root[i] != p
+    in_prefix = false
+    relative_dir_parts += p
+  endif
+  i += 1
+endforeach
+relative_dir = join_paths(relative_dir_parts) + '/'
+
+if cc.has_argument('-fmacro-prefix-map=/foo=')
+  add_project_arguments('-fmacro-prefix-map=@0@='.format(relative_dir), language: 'c')
+endif
+
 math = cc.find_library('m')
 threads = dependency('threads')
 pixman = dependency('pixman-1')