contrib: confusing diangostic message without glslang headers

When running make in contribs on macosx without contribs nor prebuilt libraries:

make: Entering directory '/Users/unidan/Workspace/vlc/contrib/contrib-x86_64-apple-darwin21'
<stdin>:1:12: fatal error: 'glslang/build_info.h' file not found
 # include <glslang/build_info.h> 
           ^~~~~~~~~~~~~~~~~~~~~~
1 error generated.

The build continues after that

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