Fix demangle bug

Summary:
Avoid a forward declaration mismatch between `libiberty.h` and
`libgen.h` that dirs in modular builds (likely related to T35368090).
This defines `HAVE_DECL_BASENAME`, which is normally defined
in the `config.h` generated during the binutils build, to prevent this
conflicting declaration.

Reviewed By: igorsugak

Differential Revision: D20600924

fbshipit-source-id: 0fb6287719abb3cf52d7eda457efbe567985d1d4

folly/detail/Demangle.cpp

@@ -23,7 +23,11 @@
 // So we extract the inclusion of `<demangle.h>` which includes `<libiberty.h>`
 // to here, isolating it.
 #if FOLLY_DETAIL_HAVE_DEMANGLE_H
+// Work around an issue with conflicting `basename` definitions in glibc and
+// binutils headers.
+#define HAVE_DECL_BASENAME 1
 #include <demangle.h> // @manual
+#undef HAVE_DECL_BASENAME
 #endif
 
 namespace folly {