Enforce python2 for systems that use python3 as their default.

Patch by: Dan Liew

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@200413 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed ninja build issues relating to use of $(DESTDIR)

We use ${DESTDIR} syntax now instead of $(DESTDIR) because that syntax
works both is the shell (at least it does for bash) and for make (at
least it does for GNU Make)

Patch By: Dan Liew

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@200414 91177308-0d34-0410-b5e6-96231b3b80d8
Fixed rules names so they are unique when aliases are present.

This is necessary for building with Ninja because it does not allow
duplicate rule names.

Patch by: Dan Liew

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@200415 91177308-0d34-0410-b5e6-96231b3b80d8
Updated README.TXT with information about using DESTDIR and building with Ninja.

Patch by: Dan Liew

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@200416 91177308-0d34-0410-b5e6-96231b3b80d8