math/z3: fix build on powerpc with clang

Add -fPIC to CXXFLAGS:
ld: error: relocation R_PPC_ADDR16_HA cannot be used against local symbol; recompile with -fPIC

defined in api/dll/gparams_register_modules.o
referenced by gparams_register_modules.cpp

api/dll/gparams_register_modules.o:(gparams_register_modules())

Adding -Wl,-znotext to LDFLAGS doesn't help.