Introduce $(intcmp ...) for numerical comparison

Introduce $(intcmp ...) for numerical comparison

Numbers can come from $(words ...), automatic variables such as
$(MAKELEVEL), from environment variables, or from shell output such as
through $(shell expr ...).  The $(intcmp ...) function allows
conditional evaluation controlled by numerical variables.

* NEWS: Announce this feature.
* doc/make.texi (Functions for Conditionals): Document 'intcmp'.
* src/function.c (func_intcmp): Create the 'intcmp' built-in function.
* tests/scripts/functions/intcmp: Test the 'intcmp' built-in function.