Fix arch/gcc7 compile errors

[JollyMort]

This is fixing few issues when trying to compile under Arch Linux with gcc6 or 7. Fixes taken from here

Skipped this commit because the patch was conflicting and it builds without that one.

Now builds on Arch x64. Last test fails, dunno if that was introduced with this or not.

Running tests...
Test project /home/m/git/jm/karbowanec/build/release
      Start  1: DifficultyTests
 1/11 Test  #1: DifficultyTests ..................   Passed    0.05 sec
      Start  2: hash-fast
 2/11 Test  #2: hash-fast ........................   Passed    0.01 sec
      Start  3: hash-slow
 3/11 Test  #3: hash-slow ........................   Passed    0.10 sec
      Start  4: hash-tree
 4/11 Test  #4: hash-tree ........................   Passed    0.00 sec
      Start  5: hash-extra-blake
 5/11 Test  #5: hash-extra-blake .................   Passed    0.00 sec
      Start  6: hash-extra-groestl
 6/11 Test  #6: hash-extra-groestl ...............   Passed    0.01 sec
      Start  7: hash-extra-jh
 7/11 Test  #7: hash-extra-jh ....................   Passed    0.01 sec
      Start  8: hash-extra-skein
 8/11 Test  #8: hash-extra-skein .................   Passed    0.00 sec
      Start  9: HashTargetTests
 9/11 Test  #9: HashTargetTests ..................   Passed    0.03 sec
      Start 10: SystemTests
10/11 Test #10: SystemTests ......................   Passed    7.50 sec
      Start 11: UnitTests
11/11 Test #11: UnitTests ........................***Failed  232.47 sec

91% tests passed, 1 tests failed out of 11

Total Test time (real) = 240.20 sec

The following tests FAILED:
	 11 - UnitTests (Failed)
Errors while running CTest
make[1]: *** [Makefile:84: test] Error 8
make[1]: Leaving directory '/home/m/git/jm/karbowanec/build/release'
make: *** [Makefile:23: test-release] Error 2

[seredat]

It builds without errors on Windows