MINOR: init: report the compiler version in haproxy -vv

MINOR: init: report the compiler version in haproxy -vv

Some portability issues were met a few times in the past depending on
compiler versions, but this one was not reported in haproxy -vv output
while it's trivial to add it. This patch tries to be the most accurate
by explicitly reporting the clang version if detected, otherwise the
gcc version.