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

liblll/CodeFragment.cpp

@@ -22,12 +22,18 @@
 #include "CodeFragment.h"
 
 #include <boost/algorithm/string.hpp>
-#pragma warning(push)
+
+#if defined(__GNUC__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wunused-parameter"
+#endif // defined(__GNUC__)
+
 #include <boost/spirit/include/support_utree.hpp>
-#pragma warning(pop)
+
+#if defined(__GNUC__)
 #pragma GCC diagnostic pop
+#endif // defined(__GNUC__)
+
 #include <libdevcore/CommonIO.h>
 #include <libevmasm/Instruction.h>
 #include "CompilerState.h"