Switch to non-recursive Makefile
tests: minor change to improve portability

Previous syntax failed on Mac-OS X.
build: add '--enable-debian-hardening' to './configure'

report CPPFLAGS at the end of './configure'
src/calc: (temporarily) ignore the return value of fwrite

using gnulib's 'clouseout' module will check for write
errors when the program terminates.
Travis-CI: build with debian hardening flags
Travis-CI: don't get git submodules. 'bootstrap' should do it.