build: Add an option to use more conservative GResource embedding

Doing clever things with objcopy is faster, but also doesn't work on
all toolchains and architectures: in particular, Debian has had trouble
with this on arm and mips.

Developers will want to leave this option switched on for faster
incremental builds (unless their toolchain doesn't support it), but in
a distro build environment where we are compiling all of GTK every time,
the cost of potentially unreliable builds is higher than the cost of
using slower but more conservative GResource embedding.

Resolves: #5107
Signed-off-by: Simon McVittie <smcv@debian.org>
15 jobs for wip/smcv/reliable-gresource in 26 minutes and 8 seconds (queued for 1 second)