OS X build fix

[wispym.gcc@mailnull.com]

System
------

MacBook Pro

OS X Yosemite Version 10.10.4

XCode Version 6.4 (6E35b)

$ clang --version
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.4.0
Thread model: posix

Building gcc-5.2.0

Problems
--------

There were two build problems to overcome:

1) missing /usr/include directory

2) ../../../../gcc-5.2.0/libsanitizer/asan/asan_malloc_mac.cc:17:35: fatal error: CoreFoundation/CFBase.h: No such file or directory

How I fixed my build
--------------------

1) ln -s /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/usr/include /usr/

2) changed asan_malloc_mac.cc to include "CFAvailability.h" instead of CFBase.h
and copied CFAvailability.h from Xcode directly into asan directory
[ref: https://developer.apple.com/library/mac/releasenotes/General/APIDiffsMacOSX10_9/CoreFoundation.html]

Extra information, for completeness
-----------------------------------

gmp
./configure --prefix=/usr/local/gcc

mpfr
./configure --{prefix,with-gmp}=/usr/local/gcc

mpc
./configure --{prefix,with-gmp,with-mpfr}=/usr/local/gcc

gcc
../gcc-5.2.0/configure --{prefix,with-{gmp,mpfr,mpc}}=/usr/local/gcc --enable-languages=c,c++

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