Force usage of GCC on MacOS for #2

tests/nim.cfg

@@ -1 +1,14 @@
 -p:"../src"
+
+# TODO FIXME
+# Default compiler on Mac is Clang.
+# Currently it does not compile due to "Flexible array member with non-trivial destruction"
+# See https://github.com/status-im/nimbus/issues/2
+# As a workaround, forces GCC-7 on Mac
+# GCC-7 is available through Homebrew
+
+@if macosx:
+  cc:"gcc"
+  gcc.cpp.exe:"/usr/local/bin/g++-7"
+  gcc.cpp.linkerexe:"/usr/local/bin/g++-7"
+@end