Conditionally include fmt/format.h (#1551)

Summary:
Addresses - at least partly - issue #1550

We'd like to limit our exposure to additional libraries. Fmt is a new dependency. It doesn't look like we need it in actuality; gating the include based on whether the file exists or not makes it possible for clients (specifically React Native for Windows) to not need to fork Folly.

Pull Request resolved: #1551

Reviewed By: yfeldblum

Differential Revision: D27531237

Pulled By: Orvid

fbshipit-source-id: 340a7ff49be81872aaf23044945a1e8ecd157546

folly/Range.h

@@ -41,7 +41,9 @@
 #include <string_view> // @manual
 #endif
 
+#if __has_include(<fmt/format.h>)
 #include <fmt/format.h>
+#endif
 
 #include <folly/CpuId.h>
 #include <folly/Likely.h>