pcre: add support for 16 bits and 32 bits variants

Until now, we have been building only the 8bits PCRE variant. However,
Qt5 requires the 16bits variant. This commit therefore adds support to
build the 16bits and 32bits variants of PCRE. In order to preserve
backward compatibility, the 8bits variant is automatically chosen if
no specific variant is defined.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>