firefox: implement install
gcc3: fix regression, causing build on debian and arch to fail

introduced in 66f57ca1b1e5c6.
the code was added to fix a mismatch between musl's and gcc's
stddef.h, when i used an old 32 bit sabotage chroot to bootstrap
a recent sabotage.
it broke some glibc platforms that don't supply an own stddef.h,
but rely on gcc's copy in some arch specific subdir.
with recent musl, this mismatch seems to have been fixed, as it
build perfectly fine without this.

fixes #136
xorg-server: make a symlink /etc/X11 -> /share/X11

closes #134