Added guards for unknown pragmas

Removed push/pop if there was no change to the warnings *in the same
file* for a given compiler. This assumes the imported boost headers use
a warning stack themselves.

The pragmas don't seem to be required anymore, but were not removed to
mantain compatibility with older versions of the boost library.

Compiled with
- clang version 6.0.1 (tags/RELEASE_601/final)
- gcc (GCC) 8.1.1 20180531
 against:
- libboost 1.67.0-5

libevmasm/KnownState.h

@@ -29,12 +29,18 @@
 #include <tuple>
 #include <memory>
 #include <ostream>
-#pragma warning(push)
-#pragma GCC diagnostic push
+
+#if defined(__clang__)
+#pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wredeclared-class-member"
+#endif // defined(__clang__)
+
 #include <boost/bimap.hpp>
-#pragma warning(pop)
-#pragma GCC diagnostic pop
+
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif // defined(__clang__)
+
 #include <libdevcore/CommonIO.h>
 #include <libdevcore/Exceptions.h>
 #include <libevmasm/ExpressionClasses.h>