[polly] move detection of polly libs to LLVM

[Sebastian Pop]

Sebastian Pop wrote:
> Chandler Carruth wrote:
> > CMake should be perfectly capable of sustaining transitive deps so that if
> > the tolo deps on the archive, the tool links against the libraries.
> 
> Good to know.
> I will try to keep the toplevel LLVM cmake clean.

For reference, here is how this works in the last patchset that I sent out:

In polly's toplevel CMakeFile, we collect in POLLY_LINK_LIBS all the libs we
need to link with polly.  Then, we make that variable available to the parent
scope, that is llvm/tools, such that we can link the tools against polly and all
the dependent libs:

+# Set the variable POLLY_LINK_LIBS in the llvm/tools/ dir.
+set(POLLY_LINK_LIBS ${POLLY_LINK_LIBS} PARENT_SCOPE)

>From CMake docs:

If PARENT_SCOPE is present, the variable will be set in the scope above the
current scope. Each new directory or function creates a new scope. This command
will set the value of a variable into the parent directory or calling function
(whichever is applicable to the case at hand)

Sebastian
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