Fix build on Mac OSX Sierra

Summary:
There are two changes here.
The first is to eliminate `detail::DEFAULT_CLOCK_ID` from `Benchmark.[cpp|h]` as Sierra defines `clockid_t` as an enum type, which means that calling `clock_gettime(detail::DEFAULT_CLOCK_ID` would fail, because the enums are incompatible. As this was being used as a default, but is not actually changable anywhere, I just got rid of `detail::DEFAULT_CLOCK_ID` entirely.
The second is to move `portability/BitsFunctexcept.cpp` into `libfollybase_la_SOURCES`, because it's needed for generating the fingerprint tables.

Reviewed By: yfeldblum

Differential Revision: D4004843

fbshipit-source-id: b2a9c33f8e516d8eb3cdc5ab093f4946ac9ed37e

Author: Orvid

folly/Benchmark.cpp

@@ -241,7 +241,7 @@ static double runBenchmarkGetNSPerIteration(const BenchmarkFun& fun,
   static uint64_t resolutionInNs = 0;
   if (!resolutionInNs) {
     timespec ts;
-    CHECK_EQ(0, clock_getres(detail::DEFAULT_CLOCK_ID, &ts));
+    CHECK_EQ(0, clock_getres(CLOCK_REALTIME, &ts));
     CHECK_EQ(0, ts.tv_sec) << "Clock sucks.";
     CHECK_LT(0, ts.tv_nsec) << "Clock too fast for its own good.";
     CHECK_EQ(1, ts.tv_nsec) << "Clock too coarse, upgrade your kernel.";

folly/Benchmark.h

@@ -52,13 +52,6 @@ inline bool runBenchmarksOnFlag() {
 
 namespace detail {
 
-/**
- * This is the clock ID used for measuring time. On older kernels, the
- * resolution of this clock will be very coarse, which will cause the
- * benchmarks to fail.
- */
-enum Clock { DEFAULT_CLOCK_ID = CLOCK_REALTIME };
-
 typedef std::pair<uint64_t, unsigned int> TimeIterPair;
 
 /**
@@ -116,7 +109,7 @@ inline uint64_t timespecDiff(timespec end, timespec start,
  */
 struct BenchmarkSuspender {
   BenchmarkSuspender() {
-    CHECK_EQ(0, clock_gettime(detail::DEFAULT_CLOCK_ID, &start));
+    CHECK_EQ(0, clock_gettime(CLOCK_REALTIME, &start));
   }
 
   BenchmarkSuspender(const BenchmarkSuspender &) = delete;
@@ -149,7 +142,7 @@ struct BenchmarkSuspender {
 
   void rehire() {
     assert(start.tv_nsec == 0 || start.tv_sec == 0);
-    CHECK_EQ(0, clock_gettime(detail::DEFAULT_CLOCK_ID, &start));
+    CHECK_EQ(0, clock_gettime(CLOCK_REALTIME, &start));
   }
 
   template <class F>
@@ -176,7 +169,7 @@ struct BenchmarkSuspender {
 private:
   void tally() {
     timespec end;
-    CHECK_EQ(0, clock_gettime(detail::DEFAULT_CLOCK_ID, &end));
+    CHECK_EQ(0, clock_gettime(CLOCK_REALTIME, &end));
     nsSpent += detail::timespecDiff(end, start);
     start = end;
   }
@@ -203,9 +196,9 @@ addBenchmark(const char* file, const char* name, Lambda&& lambda) {
     unsigned int niter;
 
     // CORE MEASUREMENT STARTS
-    auto const r1 = clock_gettime(detail::DEFAULT_CLOCK_ID, &start);
+    auto const r1 = clock_gettime(CLOCK_REALTIME, &start);
     niter = lambda(times);
-    auto const r2 = clock_gettime(detail::DEFAULT_CLOCK_ID, &end);
+    auto const r2 = clock_gettime(CLOCK_REALTIME, &end);
     // CORE MEASUREMENT ENDS
 
     CHECK_EQ(0, r1);

folly/Makefile.am

@@ -387,6 +387,7 @@ libfollybase_la_SOURCES = \
 	Format.cpp \
 	FormatTables.cpp \
 	MallctlHelper.cpp \
+	portability/BitsFunctexcept.cpp \
 	StringBase.cpp \
 	String.cpp \
 	Unicode.cpp
@@ -448,7 +449,6 @@ libfolly_la_SOURCES = \
 	detail/SocketFastOpen.cpp \
 	MacAddress.cpp \
 	MemoryMapping.cpp \
-	portability/BitsFunctexcept.cpp \
 	portability/Dirent.cpp \
 	portability/Environment.cpp \
 	portability/Fcntl.cpp \