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