[patch #8186] ax_pthread adds -pthread when it should not on OSX when compiling with clang

Follow-up Comment #10, patch #8186 (project autoconf-archive):

I am seeing the same issue that Brian mentioned.
Using this patch on OSX 10.9, with CC set to either gcc or clang (it appears
they are both llvm front-ends):

alistair@gibi:~$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr
--with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include/c++/4.2.1
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin13.0.0
Thread model: posix

alistair@gibi:~$ clang --version
Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
Target: x86_64-apple-darwin13.0.0
Thread model: posix

When I use the patched ax_pthread macro it adds -pthread to CFLAGS.
This works just fine during the compilation phase, but when linking yields
this (misleading) warning:

clang: warning: argument unused during compilation: '-pthread'

The fix suggested by Brian does indeed make this go away.
My *guess* as to why this fixes the problem is because while clang will accept
-pthread during compilation, it is not needed; whereas during linking it
throws a warning if it is used.

From man pthread(3):
INSTALLATION
     The default system libraries include pthread functions.  No additional
libraries or CFLAGS are necessary to use these interfaces.

Hope this helps.

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