wvstreams: Pass -fPIC to CFLAGS when building PIC objects

The argp bundled dependency which is linked to the libwvutils.so
shared library, wasn't built with -fPIC.
MIPS will refuce to link a non-PIC library with a shared one.

We fix this problem by appending -fPIC to CFLAGS and build all the
shared objects with -fPIC.

[Peter: use +=]
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>