The motivation for this change is to fix linking issues on OSX.
However this only partially fixes linking issues (the uninstrumented
tests and a few others won't succesfully link yet).
This change introduces a struct of function pointers
(``fuzzer::ExternalFuntions``) which when initialised will point to the
optional functions if they are available. Currently these
``LLVMFuzzerInitialize`` and ``LLVMFuzzerCustomMutator`` functions.
Two implementations of ``fuzzer::ExternalFunctions`` constructor are
provided one for Linux and one for OSX.
The OSX implementation uses ``dlsym()`` because the prior implementation
using weak symbols does not work unless the additional flags are passed
to the linker.
The Linux implementation continues to use weak symbols because the
``dlsym()`` approach does not work unless additional flags are passed
to the linker.
Differential Revision: http://reviews.llvm.org/D20741
llvm-svn: 271491
