We already fixed a couple of these in previous patches - this one should
replace all remaining uses of single brackets ("[") by double brackets.
Also, use arithmetic evaluation instead of conditional expressions where
appropriate and make use of "-z" and "-n" instead of comparing variables
to empty strings.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>

* common.sh is included on build time
* most functions are copied from makepkg

This build system overhaul allows for adding (define-style) macros to
our scripts. All source files are now suffixed with ".in" to clarify
that they might contain unprocessed defines. The Makefile provides a new
rule to preprocess source files and generate proper output scripts.

Also, add a "@pkgdatadir@" define (as used in GNU Autotools) and use it
instead of hardcoded paths to "/usr/share/devtools" everywhere. We
missed this when adding PREFIX support to the build system in commit
35fc83ce.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>