Use -fmacro-prefix-map to strip build path

Because meson does not provide a simple way to get the relative build
path, it is computed with a pair of foreach loops. As meson does not
have a simple way to compute string length (except via underscorify
and 63 split operations), the build script uses a shell command
instead.

If the compiler does not suppot -fmacro-prefix-map, then fall back
to passing in the relative path prefix, and use its length to offset
the uses of __FILE__ in log messages so that the build path is at
least still not included in the logs. This is significantly more
efficient than calling _sway_strip_path.

meson.build

@@ -13,8 +13,6 @@ project(
 
 add_project_arguments(
 	[
-		'-DSWAY_SRC_DIR="@0@"'.format(meson.current_source_dir()),
-
 		'-DWL_HIDE_DEPRECATED',
 		'-DWLR_USE_UNSTABLE',
 
@@ -142,6 +140,44 @@ if git.found()
 endif
 add_project_arguments('-DSWAY_VERSION=@0@'.format(version), language: '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) + '/'
+
+# Strip relative path prefixes from the code if possible, otherwise hide them.
+if cc.has_argument('-fmacro-prefix-map=/prefix/to/hide=')
+	add_project_arguments(
+		'-fmacro-prefix-map=@0@='.format(relative_dir),
+		language: 'c',
+	)
+else
+	add_project_arguments(
+		'-DSWAY_REL_SRC_DIR="@0@"'.format(relative_dir),
+		language: 'c',
+	)
+endif
+
+
 sway_inc = include_directories('include')
 
 subdir('include')