Allow building for the MacOSX homebrew packaging system

As far as I can this patch further builds upon paths generated by ./configure, thus not breaking building on standardized platform yet enabling the homebrew package manager for MacOSX to intergrate bgpq3 easier. 

Homebrew has an interesting approach to paths, currently I have to https://github.com/Homebrew/homebrew/pull/25136/files work with a small diff to allow for building on macosx. 

```
Eleanor:~ job$ ls -alhR /usr/local/Cellar/bgpq3/0.1.19
total 32
drwxr-xr-x  7 job  admin   238B Dec 14 19:31 .
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 ..
-rw-r--r--  1 job  wheel   4.0K May  9  2013 CHANGES
-rw-r--r--  1 job  wheel   1.4K Jan  8  2013 COPYRIGHT
-rw-r--r--  1 job  admin   219B Dec 14 19:31 INSTALL_RECEIPT.json
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 bin
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 share

/usr/local/Cellar/bgpq3/0.1.19/bin:
total 88
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 .
drwxr-xr-x  7 job  admin   238B Dec 14 19:31 ..
-r-xr-xr-x  1 job  admin    40K Dec 14 19:31 bgpq3

/usr/local/Cellar/bgpq3/0.1.19/share:
total 0
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 .
drwxr-xr-x  7 job  admin   238B Dec 14 19:31 ..
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 man

/usr/local/Cellar/bgpq3/0.1.19/share/man:
total 0
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 .
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 ..
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 man8

/usr/local/Cellar/bgpq3/0.1.19/share/man/man8:
total 16
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 .
drwxr-xr-x  3 job  admin   102B Dec 14 19:31 ..
-rw-r--r--  1 job  admin   7.1K Dec 14 19:31 bgpq3.8
Eleanor:~ job$
```