Port to Darwin and FreeBSD.

[splbio]

This should be a pretty clean port to OS X and FreeBSD.

Use err(3) and warn(3) instead of error(3).

Define __fpending.

Hide headers specific to Linux.

This was mostly taken from the FreeBSD port by mm@FreeBSD.org.

Add a howto build on Darwin/OS X.

[bukzor]

@jnovy "please advise"

[bukzor]

fwiw this is the only patch that homebrew uses to build pxz on OSX, but it's a very old version of the code they're working with:

diff --git a/pxz.c b/pxz.c
index b54f3fc..3e7e86a 100644
--- a/pxz.c
+++ b/pxz.c
@@ -259,7 +259,7 @@ int main( int argc, char **argv ) {
                        exit(EXIT_FAILURE);
                }

-               m = mmap(NULL, s.st_size, PROT_READ, MAP_SHARED|MAP_POPULATE, fileno(f), 0);
+               m = mmap(NULL, s.st_size, PROT_READ, MAP_SHARED, fileno(f), 0);
                if (m == MAP_FAILED) {
                        perror("mmap failed");
                        exit(EXIT_FAILURE);