Configurable VST SDK directory

[nick87720z]

For now VST SDK dir is hardcoded to too specific paths at home dir. But that's not enough convenient, as vst sdk could be packaged in some distros. E.g. some packages, like drumgizmo or muse-sequencer, allow to configure, where to search vst headers (if enabled). I have them packages too - vstsdk2.4 is still possible to find in some web archive (unlike 2.3, which is now out of all internet).
Some distros may package it in wrong way, there how I rewrote existing gentoo ebuild: https://github.com/nick87720z/gentoo-overlay/blob/master/media-libs/vst-sdk/vst-sdk-2.4-r1.ebuild

[yust1n]

https://aur.archlinux.org/cgit/aur.git/plain/add-vstsdk-location-var.patch?h=radium7
This patch adds -I${RADIUM_VST2SDK_PATH} wherever it is needed.
All you have to do is to define it before compiling.