BUILD: makefile: clean the wolfssl include and lib generation rules

BUILD: makefile: clean the wolfssl include and lib generation rules

The default include paths for wolfssl didn't match the explicit pattern
one. This was causing some confusion about what to look for, complexifying
the rules and making /usr/local/include to be automatically included if a
path was not set.

Let's just proceed as we usually do, i.e. pass -I only when a path is
specified, so that it works similarly to openssl. Let's also simplify
the LDFLAG rule at the same time.

This may be backported to 2.7 to ease testing of wolfssl.