configure speedup proposal: add --assume-c99 and --assume-posix2008 flags

[John Spencer]

there are many configure scripts out there that still check for things 
that are standard since at least 10 years, and doing this extensively 
and over and over (people building software themselves usually build 
more than one package) consumes a lot of time (especially due to the 
non-parallel nature of configure scripts).
often configure scripts take longer to finish than compiling the source 
code itself on decent machines with multiple cpu cores.

having an option like --assume-c99 could provide a shortcut so all 
checks like

- have stdint.h
- have snprintf()
- etc

and for posix
- sizeof int == 4
- have select()
- etc

could be skipped and assumed to be always true.

only functions which are known to be non-conforming on some platforms 
(like the broken strerror_r() semantics of GLIBC which conflict with 
POSIX when certain CPPFLAGS are used), and special platform specific 
things like linux extensions needed to be checked separately using this 
mechanism.

to me, this sounds like a simple way to make configure runs at least 50% 
faster on most modern platforms, while not impacting legacy systems 
negatively at all.

having configure run much faster could also lead to a better public 
perception of autoconf in general.

as for how to implement, maybe these flags could just set the 
appropriate autoconf cache variables to true.

regards,
--JS