AU and AUv3 plugins only support version numbers with up to 3 parts.

This change allows multiple versions of JUCE to be installed to the same
CMAKE_INSTALL_PREFIX, avoiding conflicting paths to juceaide.

This change helps to avoid ambiguity in includes when building projects
which contain more than one "BinaryData.h".

Once JUCE has been built, you can use the following line to include
the targets from the build tree (replace `JUCE_BUILD_DIR` as
appropriate).
```
include("${JUCE_BUILD_DIR}/JUCEExportConfig.cmake")
```