mail/mailsync: Fix build with Clang 6.0.0

exit function declared in autoconf 2.59 configure scripts conflicts
with Clang 6 because stdlib.h declares it with an attribute for C++11
or later. This is fixed in later versions of autoconf 2.60.

• Add USES=autoreconf to regenerate configure, new version of autoconf no longer declares the exit function. [1]
• Merge the changes from files/patch-configure and post-patch taget into acinclude/ac_with_{cclient,openssl}.m4

[1] http://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=a71c24a704ec0570ba99be909fffbc044d50908b

Reviewed by: koobs, mat
Approved by: koobs (mentor)
Differential Revision: https://reviews.freebsd.org/D15532
MFH: 2018Q2