Use --config-settings in external libraries list in installation.rst

[nulano]

These two references to the build flags were missed in #7171.

[hugovk]

Thanks!

[radarhere]

I feel a bit weird about

Starting with Pillow 3.0.0, libjpeg is required by default, but may be disabled with the -C jpeg=disable flag.

since "libjpeg is required by default" is since Pillow 3.0.0, but -C jpeg=disable isn't until much later. My suggestion is to split this into two sentences? nulano#24