Enable hardening

[jabl]

Adds some hardening flags that will hopefully make it easier to catch programming mistakes, or stop exploit attempts.

See e.g. https://wiki.debian.org/Hardening https://fedoraproject.org/wiki/Changes/Harden_All_Packages

There are some additional hardening features that can be given as linker flags (LDFLAGS):

-Wl,-z,relro,now

but these are not enabled by this commit since they can cause problems for users who load libmunge.so at runtime.