FIX: configure.ac: AS_IF for date(1)
configure.ac: Require a2x when building from git

Otherwise make in the doc/man{1,5} will fail as the manpages are
missing.

Backported from libquvi (6cdebfd, 6612228).

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
--version: Add CC and CFLAGS to output

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
--version: libquvi: Add QUVI_VERSION_BUILD_CC_FLAGS to output

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>