build: Build and check (ice-9 popen) only when --enable-posix and HAVE_FORK.

build: Build and check (ice-9 popen) only when --enable-posix and HAVE_FORK.

Fixes <http://bugs.gnu.org/13848>.
Reported by Jan Schukat <shookie@email.de>.

* configure.ac: Rename `HAVE_FORK' conditional to `BUILD_ICE_9_POPEN'.
  Set it when both $enable_posix and $ac_cv_func_fork are true.
* libguile/posix.c (scm_init_posix): Add the `fork' feature.
* doc/ref/api-options.texi (Common Feature Symbols): Add `fork'.
* doc/ref/posix.texi (Pipes): Add footnote mentioning the `fork'
  feature.
* module/Makefile.am (SCRIPTS_SOURCES): Make `scripts/autofrisk.scm' and
  `scripts/scan-api.scm' conditional on `BUILD_ICE_9_POPEN'.
* test-suite/tests/popen.test (if-supported): New macro.
  Wrap body in `if-supported'.