This was needed on my docker container running Debian Jessie. This is the error without it:
```text
Package libgsf-1 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libgsf-1.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libgsf-1', required by 'vips', not found
gyp: Call to 'PKG_CONFIG_PATH=":$PKG_CONFIG_PATH:/usr/local/lib/pkgconfig:/usr/lib/pkgconfig" pkg-config --libs vips' returned exit status 1. while trying to load binding.gyp
```
