Replace FOLLY_HAVE_LIBDWARF_DWARF_H with __has_include

Summary:
[Folly] Replace `FOLLY_HAVE_LIBDWARF_DWARF_H` with `__has_include`.

Now that folly only targets which support `__has_include`.

Reviewed By: Orvid

Differential Revision: D9281453

fbshipit-source-id: f416a6e4eb25ec31f86c705b7e9aafb1064e876a

folly/experimental/symbolizer/Dwarf.cpp

@@ -18,10 +18,9 @@
 
 #include <type_traits>
 
-// FOLLY_HAVE_LIBDWARF_DWARF_H is only ever set for autoconf-based builds.
 // We can delete this #if check once we completely deprecate and remove
 // the autoconf build.
-#if FOLLY_HAVE_LIBDWARF_DWARF_H
+#if __has_include(<libdwarf/dwarf.h>)
 #include <libdwarf/dwarf.h>
 #else
 #include <dwarf.h> // @manual