libzmq used to switch off pedantic checks when using tweetnacl. As
this is now the default, that means pedantic checks are always off.
This is not good.

Solution: in tweetnacl.c alone, use a GCC pragma to disable sign
comparison warnings. We could also clean the code up yet this is
simpler. In other code, we still want those warnings, hence I've
used a pragma rather than global compile option.

Second, use -Wno-long-long all the time, as this warning does not
work with a pragma.

I removed code that set -wno-long-long, for MinGW and Solaris.

Related problem 2: --with-relaxed is badly named

This option switches off pedantic checks, so should be called
--disable-pedantic. 'with' is for optional packages.