make folly/detail/FunctionalExcept.* work if FOLLY_HAVE_BITS_FUNCTEXCEPT_H is set or not

Summary: When doing android development, we need to compile with
FOLLY_HAVE_BITS_FUNCTEXCEPT_H set for some flavors, and
FOLLY_HAVE_BITS_FUNCTEXCEPT_H unset for others (primarily for tests
run in the local (xcode) development platform).  This diff adds some
more conditionals so it all works.

In this repro, these files are never used, so this is more or less a
noop, but this is the upstream so I'm making these changes here.

Reviewed By: @Gownta

Differential Revision: D2202107

folly/detail/FunctionalExcept.cpp

@@ -14,6 +14,13 @@
  * limitations under the License.
  */
 
+#include <folly/Portability.h>
+
+// If FOLLY_HAVE_BITS_FUNCTEXCEPT_H is set, this file compiles to
+// nothing.
+
+#if !FOLLY_HAVE_BITS_FUNCTEXCEPT_H
+
 #include <folly/detail/FunctionalExcept.h>
 
 #include <stdexcept>
@@ -39,3 +46,5 @@ void __throw_bad_alloc() {
 #endif
 
 FOLLY_NAMESPACE_STD_END
+
+#endif

folly/detail/FunctionalExcept.h

@@ -19,6 +19,8 @@
 
 #include <folly/Portability.h>
 
+#if !FOLLY_HAVE_BITS_FUNCTEXCEPT_H
+
 FOLLY_NAMESPACE_STD_BEGIN
 
 FOLLY_NORETURN void __throw_length_error(const char* msg);
@@ -31,4 +33,8 @@ FOLLY_NORETURN void __throw_bad_alloc();
 
 FOLLY_NAMESPACE_STD_END
 
+#else
+#error This file should never be included if FOLLY_HAVE_BITS_FUNCTEXCEPT_H is set
+#endif
+
 #endif