Flag -Wno-unused-but-set-variable unsupported by clang

[wenkokke]

Compilation following the instructions in the README fails on macOS, and probably with any operating system which primarily uses clang:

g++ -c -o main.obj main.cpp -MMD -Wall -Wextra -Werror -Wno-deprecated -Wno-unused-const-variable -std=c++0x -O3  -I../../common -I../../glpk-4.60/installed/include -I../../reluplex -I../../nnet -I.
In file included from main.cpp:18:
../../reluplex/Reluplex.h:3415:14: error: private field '_numOutOfBoundFixesViaBland' is not used [-Werror,-Wunused-private-field]
    unsigned _numOutOfBoundFixesViaBland;
             ^
1 error generated.

The flag -Wno-unused-but-set-variable should be replaced by -Wno-unused-private-field on these platforms.