pgsql: Fallback to clang in PATH with meson

Fallback to clang in PATH with meson

Some distributions put clang into a different path than the llvm
binary path.

For example, this is the case on NixOS / nixpkgs, which failed to find
clang with meson before this patch.

Author: Wolfgang Walther
Reviewed-by: Nazir Bilal Yavuz, Alvaro Herrera, Peter Eisentraut
Reviewed-by: Tristan Partin
Discussion: https://www.postgresql.org/message-id/ca8f37e1-a2c3-40e2-91f6-59c3d3652ad4@technowledgy.de
Backpatch: 16-, where meson support was added

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/4d8de281b5834c8f5e0be6ae21e884e69dffd4ce

Modified Files
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meson.build | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)