This commit matches sway's 2dc4978d8af326c310057ca8fd22a4c7f5d09335.
To help ensure a reproducible build (when debug info is disabled),
the meson build script now uses the -fmacro-prefix-map command line
argument supported by GCC to strip the build-path dependent bytes
of each __FILE__ string used by wlr_log and related functions.
A rather ugly algorithm is used to compute the relative path between
the build and source folders, because meson has no specific function
for this.
When the compiler does not support -fmacro-prefix-map, fall back
to shifting the start of each __FILE__ string by the length of the
relative path to the source directory.
bb05617414