Allow jobserver style to be forced to "pipe"

Allow jobserver style to be forced to "pipe"

Some systems that support mkfifo() don't support the usage that GNU
make wants.  Provide a way to force using "pipe" jobserver mode even
when mkfifo() is available.

* src/makeint.h (MK_OS_HURD): Define if we're on Hurd.
(JOBSERVER_USE_FIFO): Define if we have mkfifo() and we're NOT on
Hurd.
* src/main.c (main): Test JOBSERVER_USE_FIFO not HAVE_MKFIFO.
* src/posixos.c (jobserver_setup): Ditto.
* maintMakefile: Create a config check test for forcing "pipe" mode.
* tests/run_make_tests.pl: Show discovered FEATURES in verbose mode.