zsh: use the correct target pcre-config

zsh configure script mis-detects the location of pcre-config, and may use that
of host-pcre or that of the host instead of the target. Make configure use the
correct pcre-config.

Thanks to Thomas Petazzoni for pointing me at the right direction.

Fixes:
http://autobuild.buildroot.net/results/b12/b12aadf3016dab0941cae780fa9ee7bf9be8fcda/
http://autobuild.buildroot.net/results/8e8/8e8b12dbca4ef8cac2e5f33cd3cdcddd8e849840/
http://autobuild.buildroot.net/results/473/473197ec8e323ea7ae0e29e6363525dc42f9da2c/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>