This macro cannot assume that the location of install-sh and mkinstalldirs

This macro cannot assume that the location of install-sh and mkinstalldirs
is in ${srcdir}/autoconf because that is only true if the project is LLVM.
For other projects (e.g. sample), we don't want to have to distribute the
mkinstalldirs or install-sh programs because it opens a window of breakage
for projects. So, this change requires that the llvm_src variable be set
up via another AC_CONFIG_COMMANDS call. For LLVM this is done in the
configure.ac. For projects its done in the LLVM_CONFIG_PROJECT macro.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20304 91177308-0d34-0410-b5e6-96231b3b80d8