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