[README.md] add guide for OS X

"AC_LIB_PREPARE_PREFIX is m4_require'd but not m4_defun'd" 
This macro is GNU specific and won't compile with OS X gettext.
[README.md] add guide for OS X

Fix formatting and call our `AC_LIB_HAVE_LINKFLAGS`
Merge pull request #131 from ChrisLundquist/patch-2

[README.md] add guide for OS X