build: fix broken boost chrono check on some platforms

If clock_gettime is implemented outside of libc (librt in this case), configure
would fail when testing boost. Since clock_gettime is not present on all OSs,
boost only uses it when it can. Check for it in librt and add it to LIBS if
found, but don't fail if it's not (since boost won't be expecting it in this
case).

Also, reverse the link order as necessary for static libs.

Note that it's possible that there are other similar cases for boost, which may
be handled the same way.
gitian: remove unneeded option after last commit
Merge pull request #4571

a7ec027 gitian: remove unneeded option after last commit (Cory Fields)
460b32d build: fix broken boost chrono check on some platforms (Cory Fields)
qt: Use fixed-point arithmetic in amount spinbox

Fixes various issues and cleans up code

- Fixes issue #4500: Amount widget +/- has floating point rounding artifacts
- Amount box can now be emptied again, without clearing to 0

Also aligns the amount to the right, as in other places.
qt: Remove unused functions from BitcoinUnits

Remove two functions that are now unused.
ui: Make sure sendcoinsentry signals only connected once

Move signal connections to constructor where possible.