linux: Enable PIE

linux: Enable PIE

PIE (position-independent executables) is a security feature
that has been made reasonably cheap by recent improvements in
GCC; as a result, more and more Linux distributions are pushing
for its adoption.

Unfortunately, PIE and PIC are not compatible, so we have to
manage the CFLAGS and LDFLAGS used to compile the library and
the executable indipendently.