Add and rearrange building of libraries

There's a need for a target that will build all of libcrypto, so let's
add 'build_libcrypto' that does this.  For ortogonality, let's also
add 'build_libssl'.  Have both also depend on 'libcrypto.pc' and
'libssl.pc' so those get built together with the libraries.
This makes 'all' depend on fewer things directly.

Reviewed-by: Tim Hudson <tjh@openssl.org>