Speedup in the working state

[Full Decent]

I spend a good part of my life watching lines like this fly by:

checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes

I would like to propose a possible optimization to make the most
common "everything's OK" case move a lot faster. The most basic way is
by grouping as many possible simple checks into one program and
compiling and running that program. Then compare this program's output
to the expected output and if that fails, resort to the existing
method.

Since autoconf is more complicated, checking whether certain things
exist while producing non-fatal results if they aren't, there's
another possibility. Cache all the autoconf findings on the first run.
Each time autoconf is run, confirm that settings haven't changed, by
compiling a simple program and running and comparing output.

-- Full Decent