libffi: backport incorrect detection of selinux

libffi: backport incorrect detection of selinux

This patch unbreaks ghci on GRSEC kernels hardened with
TPE (Trusted Path Execution) protection.

TPE forbids mmap('rwx') files opened for writes:
    fd = open (a_file_in_tmp, O_RDWR);
    mmap (...,               PROT_READ | PROT_WRITE | PROT_EXEC, fd);

while allows anonymous RWX mappings:
    mmap (...MAP_ANONYMOUS , PROT_READ | PROT_WRITE | PROT_EXEC, -1);

Thanks to klondike for finding it out.

The result of a horrible typo.

(unreleased yet) upstream also has the patch:

http://github.com/atgreen/libffi/commit/eaf444eabc4c78703c0f98ac0197b1619c1b1bef