Including Dwarf symbolizer only if Elf is available

Summary:
Folly builds are broken in Legocastle MacOS because there's no Elf
support, but, since libdwarf is found (`-- Setting FOLLY_HAVE_DWARF: TRUE`),
folly tries to compile Dwarf symbolizer which depends on a few symbols from
Elf, which is not available, failing the compilation:

https://www.internalfb.com/intern/sandcastle/log/?instance_id=4503600026654665&step_id=4503603563228601&step_index=49&name=Build%20folly

Adding a check to make sure both Dwarf and Elf are available before compiling
Dwarf symbolizer.

See post for more details

https://fb.workplace.com/groups/fbcode/permalink/3847861398584004

Reviewed By: chadaustin

Differential Revision: D27328940

fbshipit-source-id: b06b852a8819275f63c57eb921562ecf37d0e63f

folly/experimental/symbolizer/Dwarf.cpp

@@ -22,7 +22,7 @@
 #include <folly/Optional.h>
 #include <folly/portability/Config.h>
 
-#if FOLLY_HAVE_DWARF
+#if FOLLY_HAVE_DWARF && FOLLY_HAVE_ELF
 
 #include <dwarf.h>
 

folly/experimental/symbolizer/Dwarf.h

@@ -28,7 +28,7 @@
 namespace folly {
 namespace symbolizer {
 
-#if FOLLY_HAVE_DWARF
+#if FOLLY_HAVE_DWARF && FOLLY_HAVE_ELF
 
 namespace detail {