Allow compiling and using as DLL in windows (#502)

* printf.h fixed to compile clean - need to check whether this is the right
thing to do

* fix warnings and errors in test compiles with BUILD_SHARED_LIBS

* did requested changes and added one change to allow all tests to succeed
in windows DLL

test/format-impl-test.cc

@@ -26,6 +26,7 @@
  */
 
 #define FMT_NOEXCEPT
+#undef FMT_SHARED
 #include "test-assert.h"
 
 // Include *.cc instead of *.h to test implementation-specific stuff.