Add optional clang compile mode

Summary:
clang is an alternate compiler based on llvm.  It produces
nicer error messages and finds some bugs that gcc doesn't, such as the
size_t change in this file (which caused some write return values to be
misinterpreted!)

Clang isn't the default; to try it, do "USE_CLANG=1 make" or "export
USE_CLANG=1" then make as normal

Test Plan: "make check" and "USE_CLANG=1 make check"

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D7899

table/table_test.cc

@@ -411,12 +411,14 @@ static bool ZlibCompressionSupported() {
                              &out);
 }
 
+#ifdef BZIP2
 static bool BZip2CompressionSupported() {
   std::string out;
   Slice in = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
   return port::BZip2_Compress(Options().compression_opts, in.data(), in.size(),
                               &out);
 }
+#endif
 
 enum TestType {
   TABLE_TEST,