* unity builds: correct integer ceiling division
* edge case failure with unity builds:
- static archive bar that gets installed, that links with another static
archive foo that does not get installed
- the number of files in static archive foo is divisible by unity_size
would yield an error with ninja:
ninja: error: 'subprojects/foo/src/libfoo.a.p/meson-generated_foo-unity1.cpp.o', needed by 'src/libbar.a', missing and no known rule to make it
* unity builds: test for build failure when #files is divisible by unity_size
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082