Build system: default include/library prefix

[michaelrsweet]

Supporting both legacy (CUPS 2.x) and current (CUPS 3.x) source code seems to be an ongoing issue. Add a configure option to control whether the header files are put under a "libcups3" subdirectory, whether the library is called "libcups3." or "libcups.", and whether we install a cups.pc file (always install cups3.pc). Something like --disable-libcups3-prefix.

[michaelrsweet]

[master 986c6ed] Prefix the libcups3 install to allow CUPS 2 and 3 to coexist (Issue #21)