Including newlib's <stdlib.h> brings in a '#define __has_include 0', so
using -U__has_include on the command line isn't going to remove it (so the
fallback doesn't happen and the test fails)
Instead use a '#undef __has_include' at the end of the prefix to excerise
this.
(newlib's <stdlib.h> is derived from FreeBSD, so the same problem will
probably be seen there)
Also forcibly undefine __has_include and test that the fallback include
check in cc.has_header() works.
This is important because all the latest compilers support it now
and we might have no test coverage at all by accident. GCC 5, ICC 17,
Clang 3.8, and VS2015 Update 2 already support it.
Since we're checking for the existence of a header, just running the
preprocessor is enough. According to my benchmarks, doing this makes the
test roughly 2x faster.
I'm sure this will be useful for other checks too in the future.
This shaves off 7% on the configure time for glib on my machine.
This also fixes the issue that we had earlier where you had to specify
any extra headers needed to resolve symbols in the header being checked
for with `prefix`.
Wolfgang Stöggl
Jussi Pakkanen
Jon Turney
Nirbheek Chauhan