Fixed nestability of TYPE_* testing macros

Some of the NT2_TEST_TYPE_* macro where not properly setup to be
used in unbraced if/else statement and other smilar contexts. Their
definition have been amended to sue the do { } while(0) idiom.
Setup a global doxygen alias file

When building the reference doxyfile of a doc, we now add the contents
of a predefined, central aliases file. This is made so we don't have to
copy/paste a potentially large number of such aliases in each and every
local reference.doxyfile.