AX_PKG_CHECK_MODULES: Add pkg-config wrapper splitting private deps

See the documentation in the macro file for a full description.
AX_COMPILER_FLAGS: Add macro enabling a consistent set of compiler flags

See the documentation comment at the top of the file for a full
description of what this macro does.
AX_CHECK_VSCRIPT: initial version

Further details can be found at <https://savannah.gnu.org/patch/?8582>.