Makefile: validate generated files as part of build

For example...

$ make
GEN checkpkg
GEN commitpkg
GEN archco
archco: line 179: unexpected EOF while looking for matching `"'
archco: line 181: syntax error: unexpected end of file
make: *** [archco] Error 2

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>