Compile ice-9/popen.scm for MinGW

[Eli Zaretskii]

Once open-process worked for me, I quickly found out that
modules/ice-9/popen.scm is not being compiled.  This is because
configure.ac explicitly excludes it for systems lacking 'fork':

   AM_CONDITIONAL([BUILD_ICE_9_POPEN],
     [test "x$enable_posix" = "xyes" && test "x$ac_cv_func_fork" = "xyes"])

Not sure how best to fix this; perhaps the test should be removed
altogether (are there any supported systems that cannot support
popen.scm?).