pcre: fixing linking error one some systems:

libmysqld.a(pcre_exec.c.o): relocation R_X86_64_32S against
`_pcre_ucd_stage1' can not be used when making a shared object;
make[2]: Leaving directory `/mnt/buildbot/build/mariadb-10.0.3'
recompile with -fPIC

Changing ADD_LIBRARY(pcre) to ADD_CONVENIENCE_LIBRARY(pcre)

pcre/CMakeLists.txt

@@ -314,9 +314,9 @@ SET(targets)
 
 # Libraries
 # pcre
-ADD_LIBRARY(pcre ${PCRE_HEADERS} ${PCRE_SOURCES} ${PROJECT_BINARY_DIR}/config.h)
+ADD_CONVENIENCE_LIBRARY(pcre ${PCRE_HEADERS} ${PCRE_SOURCES} ${PROJECT_BINARY_DIR}/config.h)
 SET(targets ${targets} pcre)
-ADD_LIBRARY(pcreposix ${PCREPOSIX_HEADERS} ${PCREPOSIX_SOURCES})
+ADD_CONVENIENCE_LIBRARY(pcreposix ${PCREPOSIX_HEADERS} ${PCREPOSIX_SOURCES})
 SET(targets ${targets} pcreposix)
 TARGET_LINK_LIBRARIES(pcreposix pcre)