This pulls in the https://github.com/darlinghq/darling-dmg/pull/41 fix
for compilation with newer GCC.
The error message this fixes is
```
osxcross/build/darling-dmg/src/main-fuse.cpp:137:28: error: no member named 'function' in namespace 'std'
int handle_exceptions(std::function<int()> func)
~~~~~^
osxcross/build/darling-dmg/src/main-fuse.cpp:137:44: error: use of undeclared identifier 'func'
int handle_exceptions(std::function<int()> func)
^
osxcross/build/darling-dmg/src/main-fuse.cpp:137:49: error: expected ';' after top level declarator
int handle_exceptions(std::function<int()> func)
^
;
3 errors generated.
```
Other changes:
* Add USE_CLANG_AS build.sh option
* build_clang.sh: Update Clang version to 3.9.1
* build_compiler_rt.sh: Adjustments for Clang 4.0 and Clang 5.0
Notes from Thomas Pöchtrager:
* The *-g++-libc++ aliases use the Clang C++ library.
* Don't use the Clang C++ library with GCC.
* You can use an installed Clang, or build your own.
* Same Clang serves both to compile OSXCross and as the cross-compiler.
* INSTALLPREFIX is specifically for installing a (native!) clang.
Other tweaks to the text:
* Broke some lines before 80 characters (found by lint check).
* Removed some trailing spaces (found by lint check).
* Put shell commands on their own, indendent lines.
* Explain GCC_VERSION, ENABLE_FORTRAN, etc. explicitly.
* Make each footnote its own paragraph, on a separate line.
* Minor tweaks to wording for style & idiom.
One behavior of cctools' libtool when invoked as ranlib is rotating archives of universal object files (such as those created by `clang -arch i386 -arch x86_64 -c ...`) into universal files containing per-architecture archives each containing single-architecture objects. From Xcode 7 until Xcode 7.3, whose cctools/ld64 are not yet open source, there was a bug where this would fail if any of the individual objects were LLVM bitcode objects rather than natively compiled ones, and incorrectly succeed with nonsensical results if such an archive contained members that were not recognizable object files at all. The fix is straightforward and this commit adds a patch to apply it while we wait for newer cctools sources to be released.
