Fix "make gcc-wall" for library code

Fix "make gcc-wall" for library code

Commit fd1677e80: "Add --enable-hardening which builds e2fsprogs with
security hardening" broken the gcc warnings mode by changing how
CFLAGS were used to build static and shared library objects.

Also shut up GCC's whining about casting the return value from
dlsym(2) to function pointers --- since due to conflicing standards,
there is no way to use dlsym() without causing gcc to whine one way or
another.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>