Merge pull request #1078 from ilovezfs/cmake-weak-symbols

formula: help CMake with 10.12 SDK on 10.11

Library/Homebrew/formula.rb

@@ -1266,7 +1266,7 @@ class Formula
   # Note: there isn't a std_autotools variant because autotools is a lot
   # less consistent and the standard parameters are more memorable.
   def std_cmake_args
-    %W[
+    args = %W[
       -DCMAKE_C_FLAGS_RELEASE=-DNDEBUG
       -DCMAKE_CXX_FLAGS_RELEASE=-DNDEBUG
       -DCMAKE_INSTALL_PREFIX=#{prefix}
@@ -1275,6 +1275,14 @@ class Formula
       -DCMAKE_VERBOSE_MAKEFILE=ON
       -Wno-dev
     ]
+
+    # Avoid false positives for clock_gettime support on 10.11.
+    # CMake cache entries for other weak symbols may be added here as needed.
+    if MacOS.version == "10.11" && MacOS::Xcode.installed? && MacOS::Xcode.version >= "8.0"
+      args << "-DHAVE_CLOCK_GETTIME:INTERNAL=0"
+    end
+
+    args
   end
 
   # an array of all core {Formula} names