Do you mean something like SynthEdit for VST ?
I've never worked with it but understood it were
a kind of plug'n-drag your own synth, without coding skills.
Some comparable apps exist:
Ingen, Alsa Modular Synth, Clam, GAlan, any more ??
If you want to code but have it easy,
there is no all-in-one solution,
because that would restrict developers,
(I know, sometimes it's nice to have no choice ;)
But there are same parts that can be combined to simplify
plugin development:
gui-creation:
gkt:
http://phat.berlios.de/
juce
http://www.rawmaterialsoftware.com/juce/
tools to make tools:
http://linux-sound.org/tools.html
Another hint for simple coding: The SndObj Library::
http://music.nuim.ie//musictec/SndObj/main.html
Good luck with all this,
Emanuel