Only remove actualy failed blocks from setBlockIndexValid
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)
Bugfix: make CCoinsViewMemPool support pruned entries in underlying cache
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.
Merge pull request #4556

29eaa31 ui: Make sure sendcoinsentry signals only connected once (Wladimir J. van der Laan)
2a05101 qt: Remove unused functions from BitcoinUnits (Wladimir J. van der Laan)
91cce17 qt: Use fixed-point arithmetic in amount spinbox (Wladimir J. van der Laan)
doc: remove any mention of SOCKS4

SOCKS4 support was removed in 0127a9b, as well as the `-socks=` option.
Fix typos in comments (missing i's).
Merge pull request #4575

ad08d0b Bugfix: make CCoinsViewMemPool support pruned entries in underlying cache (Pieter Wuille)
Merge pull request #4496

5734d4d Only remove actualy failed blocks from setBlockIndexValid (Pieter Wuille)
Add comment about never updating nTimeOffset past 199 samples

Refer to issue #4521 for details.