BUILD: add a new file "version.c" to carry version updates

BUILD: add a new file "version.c" to carry version updates

While testing fixes, it's sometimes confusing to rebuild only one C file
(e.g. a mux) and not to have the correct commit ID reported in "haproxy -v"
nor on the stats page.

This patch adds a new "version.c" file which is always rebuilt. It's
very small and contains only 3 variables derived from the various
version strings. These variables are used instead of the macros at the
few places showing the version. This way the output version of the
running code is always correct for the parts that were rebuilt.