Merging r326358: Fix libexecinfo detection on FreeBSD and NetBSD.

[bdrewery]

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r326358 | dim | 2018-02-28 12:04:21 -0800 (Wed, 28 Feb 2018) | 29 lines
Changed paths:
   M /llvm/trunk/lib/Support/CMakeLists.txt

Fix llvm-config --system-libs output on FreeBSD and NetBSD

Summary:
For various reasons, CMake's detection mechanism for `backtrace()`
returns an absolute path `/usr/lib/libexecinfo.so` on FreeBSD and
NetBSD.

Since `tools/llvm-config/CMakeLists.txt` only checks if system
libraries start with `-`, this causes `llvm-config --system-libs` to
produce the following incorrect output:

-lrt -l/usr/lib/libexecinfo.so -ltinfo -lpthread -lz -lm

Fix it by removing the path and the `lib` prefix, to make it look like a
regular short library name, suitable for appending to a `-l` link flag.

This also fixes the `Bindings/Go/go.test` test case, since that always
died with "unable to find library -l/usr/lib/libexecinfo.so".

Reviewers: chandlerc, emaste, joerg, krytarowski

Reviewed By: krytarowski

Subscribers: hans, bdrewery, mgorny, hintonda, llvm-commits

Differential Revision: https://reviews.llvm.org/D42702
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[bdrewery]

The backport commit process is unclear to me. If you want me to modify the message to another format let me know.

[alexcrichton]

Thanks!

FWIW the next steps here would be a PR to rust-lang/rust to update the submodule rev, and that should do it!