pgsql: Detect LLVM 7 without specifying binaries explicitly.

Detect LLVM 7 without specifying binaries explicitly.

Before this commit LLVM 7 was supported, but only if one explicitly
provided LLVM_CONFIG= and CLANG= paths. As LLVM 7 is the first
version that includes our upstreamed debugging and profiling features,
and as debian is planning to default to 7 due to wider architecture
support, it seems good to support auto-detecting that version.

Author: Christoph Berg
Discussion: /messages/by-id/20180912124517.GD24584@msg.df7cb.de
Backpatch: 11, where LLVM was introduced

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/99ba795d982efe2305d94573f2f4594bedaf6eab

Modified Files
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config/llvm.m4 | 4 ++--
configure | 4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)