Correct nmod_poly example.

There is no nmod_set_coeff_si() function implemented.
nmod_poly: fix signatures of the sqrt functions in the documentation.
Merge remote-tracking branch 'upstream/trunk' into trunk
Fix markup in the documentation to be able to build the manual.
Merge branch 'trunk' of https://github.com/wbhart/flint2 into trunk

Conflicts:
	nmod_poly/doc/nmod_poly.txt
Correct spelling of Ralf Hemmecke.
const fixes for nmod_poly, and a few fixes for other modules.
const fixes for fmpq_poly.
Switch to BPSW for primality testing up to 64 bits.
const fixes for fmpz_mod_poly.
Call BPSW directly in n_is_prime.
const fixes for fmpz_vec and fix to const fix for fmpz_mod_poly.
Move const into more logical position.
Fix unused variable warning.
Further fiddling with const.
const fixes for fmpz_mod_poly_factor.
Start design document.
more design work
Improve build system for cxx module.

In particular:

(Makefile.in) Autogenerate targets check-XYZ for each build dir XYZ.
(Makefile.subdirs) Allow C++ tests, add new pattern rule to compule them.

Also, add empty/dummy files cxx.h, cxx/dummy.c and cxx/test/t-cxx-compiler.cpp.
Another build system tweak.