superenv: help Autotools with 10.12 SDK on 10.11

The GNU Autotools tests for whether a given symbol is defined are
reliably coming to incorrect conclusions on 10.11 with the 10.12 SDK
in Xcode 8. This overrides its decisions by forcing the right answer
in superenv using ac_cv_func_* environment variables and setting them to
"no" on 10.11. The list of problematic symbols is from

  grep 'weak$os10.11' MacOSX.sdk/usr/lib/system/libsystem_c.tbd