Add virtual/cblas dependency

[Alex-Gorny]

Please add virtual/cblas dependency to dev-lang/rust
dev-lang/rust cannot be built if virtual/cblas is not installed.

CMake Error at cmake/modules/AddLLVM.cmake:459 (add_library):
  Cannot find source file:

    /usr/include/cblas.h

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .m .M .mm .h .hh .h++ .hm
  .hpp .hxx .in .txx
Call Stack (most recent call first):
  cmake/modules/AddLLVM.cmake:622 (llvm_add_library)
  lib/Support/CMakeLists.txt:47 (add_llvm_library)

[stefson]

gentoo bug about this: https://bugs.gentoo.org/674662

[gyakovlev]

cblas is not needed.
clean up stale symlinks on the system

in ::gentoo we carry this patch to avoid this.
https://gitweb.gentoo.org/repo/gentoo.git/tree/dev-lang/rust/files/1.32.0-fix-configure-of-bundled-llvm.patch

[Cynede]

guess patch can be ported to overlay to close this