Get rid of ax_check_compile_flag.m4

Get rid of ax_check_compile_flag.m4

The macro was too new for some of the platforms we still support.  In
particular, centos/rhel 6 and opensolaris 10.  To work around that, we
introduce our own simpler and more tailored ACL_CHECK_ADD_COMPILE_FLAGS
macro, that not only checks but also sets the flags in CFLAGS if it is
accepted.  Since this doesn't use new-and-shine autoconf features, it
should also work on the legacy platforms.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20180220202508.16201-1-steffan@karger.me>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg16515.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 6a5d10e96b9ad2f9a9472aeee8cdb7c02fe4d050)