Makefile: tune MANPREFIX configuration for FreeBSD (#1043)

We could think of the following use cases:

1. User installs git-extras from official port/pkg. man/man* structure
is quite common among different systems, but location of man/ itself can
be different. FreeBSD Ports framework setups MANPREFIX to PREFIX by
default, i.e. it's /usr/local. The idea is that software may provide
different man/man* sections, with final path being /usr/local/man/man*.
In contrast, git-extras' Makefile process simplifies the things due to
it works with section 1 only, and expects MANPREFIX to be set down to
man/man1 path. For this reason, installation via FreeBSD Ports requires
unconditional man path configuration like this:

    ifeq ($(OS), FreeBSD)
        MANPREFIX = "$(PREFIX)/man/man1"

Otherwise, the path will be incorrect. And this is how official FreeBSD
port of git-extras has been fixing this difference for many years.

2. End user installs git-extras manually. There are two sub-cases:

  2.a. User explicitly defines custom MANPREFIX. Okay, it's user's
  decision -- nothing to do here, just follow as is.

  2.b. User does not define custom paths, i.e. it's expected to be
  installed according to FreeBSD defaults. And default values in
  git-extras' Makefile forms the expected correct path.

This change supports all the cases above, with the goal to avoid wrong
man path for the most of the situations. As long as official git-extras
make process tries to support FreeBSD, this change allows to omit extra
patching for FreeBSD Ports framework.

Makefile

@@ -5,6 +5,9 @@ SHELL := bash
 OS = $(shell uname)
 ifeq ($(OS), FreeBSD)
 	MANPREFIX ?= "$(PREFIX)/man/man1"
+	ifeq ($(MANPREFIX), /usr/local)
+		MANPREFIX = "/usr/local/man/man1"
+	endif
 else
 	MANPREFIX ?= "$(PREFIX)/share/man/man1"
 endif