build fixes

[vtjnash]

No description provided.

[tkelman]

needs to be tested in a clean cygwin cross compile to ensure it doesn't bring back #16759

[tkelman]

I said this needed testing for a reason. Build changes need testing on all platforms in a clean build BEFORE merging.

make[3]: *** No rule to make target '/home/Tony/julia/usr-staging/llvm-3.7.1/build_Release+Asserts/home/Tony/julia/usr/bin', needed by '/home/Tony/julia/usr-staging/llvm-3.7.1/build_Release+Asserts/home/Tony/julia/usr/bin/LLVM-3.7.dll'.  Stop.
make[2]: *** [/home/Tony/julia/deps/srccache/llvm-3.7.1/Makefile.rules:880: install] Error 1
make[1]: *** [/home/Tony/julia/deps/llvm.mk:526: /home/Tony/julia/usr-staging/llvm-3.7.1/build_Release+Asserts.tgz] Error 2
make: *** [Makefile:81: julia-deps] Error 2

edit: #18538 fixes this