Support 8-bit & 12-bit JPEGs using the same build

Partially implements #199

This commit also implements a request from #178 (the ability to compile
the libjpeg example as a standalone program.)

README.ijg

@@ -43,7 +43,7 @@ User documentation:
   change.log        Version-to-version change highlights.
 Programmer and internal documentation:
   libjpeg.txt       How to use the JPEG library in your own programs.
-  example.txt       Sample code for calling the JPEG library.
+  example.c         Sample code for calling the JPEG library.
   structure.txt     Overview of the JPEG library's internal structure.
   coderules.txt     Coding style rules --- please read if you contribute code.