AM_MAINTAINER_MODE macro adds ability to disable rebuilding build file
(Makefile.in, configure, ...) when the source file changes. This is
important in the CI where the timestamps could get skewed and that
triggers the rebuild on every ./configure run.
Pull and use several autoconf archive convenience macros to simplify
configure.ac.
* AX_CHECK_COMPILE_FLAG(FLAG, ...) - check whether given CFLAG works
* AX_CHECK_LINK_FLAG(FLAG, ...) - check whether given LDFLAG works
* AX_CHECK_PREPROC_FLAG(FLAG, ...) - check whether give CPPFLAG works
* AX_SAVE_FLAGS/AX_RESTORE_FLAGS - save and restore *FLAGS
