Bunch of small bugfixes in last night's work.

It turns out to be a bad idea to develop decltests on a system that
doesn't implement X7; the expected failures hide genuine errors.

Because this also could hide genuine errors, temporarily disable
things glibc+(gcc/clang) don't support (_Imaginary, wchar_t in
<inttypes.h>).
X5/6/7 additions for limits.ini.

Required additional features in Maketest.py: #if/#ifdef around a test,
constraint tests of no particular constant.