Allow Solidity build to be disabled.

SolidityEndToEndTest.cpp

@@ -1,4 +1,3 @@
-
 /*
 	This file is part of cpp-ethereum.
 
@@ -22,6 +21,8 @@
  * Unit tests for the solidity expression compiler, testing the behaviour of the code.
  */
 
+#if ETH_SOLIDITY
+
 #include <string>
 #include <tuple>
 #include <boost/test/unit_test.hpp>
@@ -3627,3 +3628,4 @@ BOOST_AUTO_TEST_SUITE_END()
 }
 } // end namespaces
 
+#endif