Improves adherence to GNU makefile conventions.

Added SHELL variable. Fixes #4.
Added .SUFFIXES.
Added INSTALL/INSTALL_PROGRAM/INSTALL_DATA.
Added uninstall target.
Compiler flag '-g' is default for all builds.
Added explicit DEBUG define for debug builds.
Conditional assignment of CXX.
Update README.md

Specified timing limitations on Mac.
Added time-based sorting of source files and improved versioning.

By default source files are sorted by time stamp with the most recent files compiled first. If find, sort, or cut is unavailable, a fallback option is provided using a recursive version of GNU make's wildcard function (see http://blog.jgc.org/2011/07/gnu-make-recursive-wildcard-function.html). Closes #7.

Checks were added so that version macros are only generated when the makefile is used in a git repo that has tags.
Update Makefile comments
Add pkg-config library support via a variable in the makefile
Moved the if statement down below the DNE line
Merge pull request #9 from alekratz/master

Add pkg-config library support via a variable in the makefile
Merge pull request #1 from mbcrawfo/master

update