fix dynamic engine build

fix dynamic engine build

Builds with --dynamic-libengines are currently broken, because the rule
to build the libraries was moved out of the template that creates them:

make: *** No rule to make target 'engines/libpmem.so', needed by 'all'.  Stop.
make: *** Waiting for unfinished jobs....

Fix this by moving the rule back into the template, calling the template
to create rules only after FIO_CFLAGS has been incorporated into CFLAGS,
and using CFLAGS + -fPIC in that rule rather than FIO_CFLAGS.

Fixes: 8a2cf08d29ac ("Makefile: introduce FIO_CFLAGS")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>