Backport to 5.5

Current MySQL builds, even on Pushbuild, are not reproducible; they return
different results depending on which directory they are built from (and
Pushbuild uses several different directories). This is because absolute paths
leak into debug information, and even worse, __FILE__. The latter moves code
around enough that we've actually seen sysbench changes on the order of 4% in
some tests.

CMake seemingly insists on using absolute paths, but we can insert our own
layer between CMake and GCC to relativize all paths. Also give the right flags
to get debug information reproducible and turn off build stamping. This makes
the mysqld build 100% bit-for-bit reproducible between runs on my machine,
even when run from different directories.