Bug#871591: [src:llvm-toolchain-snapshot] FTBFS: debian/rules fails to detect gcc-7

[Katsuhiko Nishimra]

Package: src:llvm-toolchain-snapshot
Version: 1:5.0~svn305653-1
Severity: serious
Tags: patch sid
User: debian-gcc at lists.debian.org
Usertags: ftbfs-gcc-7

Dear LLVM packagers,

I've noticed that llvm-toolchain-snapshot goes FTBFS because
`debian/rules' in the packge fails to detect gcc-7 (it tries to use
gcc-7.1). The Jenkins builds seem to be failing due to the same bug.
http://llvm-jenkins.debian.net/view/Debian%20unstable/job/llvm-toolchain-binaries/architecture=amd64,distribution=unstable/898/console

Here I send a patch to fix this by ensuring the existence of
/usr/bin/g++-$(GCC_VERSION) on the build system.

Could you take a look at it please?
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