fix: return only boolean when useGlobalLibvips is used by gyp

[project0]

The debug log is compromising the gyp variable use_global_libvips and therefore breaks compilation when the detection adds additional output.

/app/node_modules/sharp # SHARP_FORCE_GLOBAL_LIBVIPS=true node -p "Boolean(require('./lib/libvips').useGlobalLibvips()).toString()" 
sharp: Detected SHARP_FORCE_GLOBAL_LIBVIPS, skipping search for globally-installed libvips
true

see also #4060 and 579cf93

[lovell]

Thanks for the PR. Given the "skipping search for globally-installed libvips" log message is somewhat confusing when SHARP_FORCE_GLOBAL_LIBVIPS is set, could this be simplified to something like the following?

+++ b/lib/libvips.js
@@ -172,7 +172,7 @@ const useGlobalLibvips = () => {
     return skipSearch(false, 'SHARP_IGNORE_GLOBAL_LIBVIPS');
   }
   if (Boolean(process.env.SHARP_FORCE_GLOBAL_LIBVIPS) === true) {
-    return skipSearch(true, 'SHARP_FORCE_GLOBAL_LIBVIPS');
+    return true;
   }
   /* istanbul ignore next */
   if (isRosetta()) {

[project0]

i guess it could, but at the same time it is a helpful information that the detection is enforced/influenced by the env variable.

[project0]

Also, given the output is used in a script we already got lucky as we never checked for the false value.

[lovell]

we already got lucky

Ha ha, yes, this is a good point well made.

I've inverted the logic slightly to inject a logger function, which I think makes it a bit easier to test. This landed as 56fae3e

Thanks again for taking the time to open a PR for this.