fix(unity): GridFire supports unity builds

GridFire can now support unity builds. This is useful for python
bindings. The main limitation is that openssl (a dependency of
libplugin) does not support unity builds. Therefore when building in a
unity build libplugin must be disabled. This is done with the
-Dunity-safe=ture flag at setup time

Doxyfile

@@ -48,7 +48,7 @@ PROJECT_NAME           = GridFire
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = v0.7.3_rc2
+PROJECT_NUMBER         = v0.7.4_rc2
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewers a