Ensure dynamic libjpeg libraries are not linked

[freakboy3742]

Fixes #9079.

Changes proposed in this pull request:

• Turns on another CMake flag (-DENABLE_SHARED=NO) to prevent libjpeg-turbo from producing dynamic libraries on iOS
• Adds a safety catch that purges any dylibs that are produced by the dependency build, so they can't be picked up by clang at link time.

I've manually audited all the binary modules that are produced as a result of this build, and I don't see any other dynamic artefacts being linked.

[hugovk]

Thanks!