pcre: fix C++ static build problem

PCRE C++ unit tests fails to build properly for static scenarios since
it's not taken account for properly in the build system.
Just disable them, they're never installed to the target.
Fixes:
http://autobuild.buildroot.net/results/970/97027b981659502ff12c345c3caef4933cddb354/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>