Update Linux compiler selection with linker

Qmake attempts to invoke the linker with the default `g++` binary even
when we changed the binary. This will compile error when using LTO.

For clang:
```
/usr/bin/ld.gold: error: /usr/lib/llvm-3.8/bin/../lib/LLVMgold.so: could
not load plugin library: /usr/lib/llvm-3.8/bin/../lib/LLVMgold.so:
cannot open shared object file: No such file or directory
```

For GCC:
```
lto1: fatal error: bytecode stream generated with LTO version 6.0
instead of the expected 4.1
```

build_scripts/qt/compilers/gcc.pri

@@ -7,14 +7,17 @@ else {
     system(g++-7 --version) {
         message('g++-7' found.)
         QMAKE_CXX = g++-7
+        QMAKE_LINK = g++-7
     }
     system(g++-8 --version) {
         message('g++-8' found.)
         QMAKE_CXX = g++-8
+        QMAKE_LINK = g++-8
     }
     system(g++-9 --version) {
         message('g++-9' found.)
         QMAKE_CXX = g++-9
+        QMAKE_LINK = g++-9
     }
 }