[sanitizer] Fix 'dyld: Symbol not found: _dyldVersionNumber' link error on old Darwin systems.

[m.ostapenko]

Summary

This patch fixes https://github.com/google/sanitizers/issues/669. On older Darwin systems (in particular, Darwin 10), dyld doesn't export _dyldVersionNumber symbol so we would have 'undefined reference' error in sanitzer library. Although sanitizers support was added to LLVM on OS X 10.7+ where _dyldVersionNumber symbol is already exported, GCC users still may want use them on older systems.

[kubamracek]

LGTM.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc

void LeakyResetEnv(const char *name, const char *name_value) {
   }
 }
 
 SANITIZER_WEAK_CXX_DEFAULT_IMPL
 bool ReexecDisabled() {
   return false;
 }
 
-extern "C" double dyldVersionNumber;
+extern "C" SANITIZER_WEAK_ATTRIBUTE double dyldVersionNumber;
 static const double kMinDyldVersionWithAutoInterposition = 360.0;
 
 bool DyldNeedsEnvVariable() {
+  // Although sanitizer support was added to LLVM on OS X 10.7+, GCC users
+  // still may want use them on older systems. On older Darwin platforms, dyld
+  // doesn't export dyldVersionNumber symbol and we simply return true.
+  if (!&dyldVersionNumber) return true;
   // If running on OS X 10.11+ or iOS 9.0+, dyld will interpose even if
   // DYLD_INSERT_LIBRARIES is not set. However, checking OS version via
   // GetMacosVersion() doesn't work for the simulator. Let's instead check
   // `dyldVersionNumber`, which is exported by dyld, against a known version
   // number from the first OS release where this appeared.
   return dyldVersionNumber < kMinDyldVersionWithAutoInterposition;
 }