As per subject, I've tried to rebuild the plugin from source following the given instructions, but it seems like CLion's Platform SDK is missing a few things that this plugin needs.
Here's the compilation report (with the latest version from the repo):
glsl4idea/src/glslplugin/components/GLSLCompiler.java
Error:(24, 37) java: package com.intellij.openapi.compiler does not exist
Error:(25, 37) java: package com.intellij.openapi.compiler does not exist
Error:(26, 37) java: package com.intellij.openapi.compiler does not exist
Error:(27, 37) java: package com.intellij.openapi.compiler does not exist
Error:(39, 38) java: cannot find symbol
symbol: class TranslatingCompiler
Error:(46, 42) java: cannot find symbol
symbol: class CompileScope
location: class glslplugin.components.GLSLCompiler
Error:(52, 62) java: cannot find symbol
symbol: class CompileContext
location: class glslplugin.components.GLSLCompiler
Error:(57, 25) java: cannot find symbol
symbol: class CompileContext
location: class glslplugin.components.GLSLCompiler
Error:(57, 97) java: cannot find symbol
symbol: class OutputSink
location: class glslplugin.components.GLSLCompiler
Error:(86, 42) java: cannot find symbol
symbol: class OutputItem
location: class glslplugin.components.GLSLCompiler
Error:(76, 16) java: cannot find symbol
symbol: class OutputItem
location: class glslplugin.components.GLSLCompiler.ExitResult
Error:(107, 17) java: cannot find symbol
symbol: class CompileContext
location: class glslplugin.components.GLSLCompiler.GLSLCompileAction
Error:(110, 34) java: cannot find symbol
symbol: class CompileContext
location: class glslplugin.components.GLSLCompiler.GLSLCompileAction
Error:(45, 5) java: method does not override or implement a method from a supertype
Error:(51, 5) java: method does not override or implement a method from a supertype
Error:(56, 5) java: method does not override or implement a method from a supertype
Error:(78, 24) java: cannot find symbol
symbol: class OutputItem
location: class glslplugin.components.GLSLCompiler.ExitResult
Error:(122, 36) java: cannot find symbol
symbol: variable CompilerMessageCategory
location: class glslplugin.components.GLSLCompiler.GLSLCompileAction
glsl4idea/src/glslplugin/actions/GLSLDeduceExpressionTypeAction.java
Error:(24, 41) java: cannot find symbol
symbol: class DataKeys
location: package com.intellij.openapi.actionSystem
Error:(33, 29) java: cannot find symbol
symbol: class PsiUtil
location: package com.intellij.psi.util
Error:(80, 41) java: cannot find symbol
symbol: variable DataKeys
location: class glslplugin.actions.GLSLDeduceExpressionTypeAction
Error:(119, 40) java: cannot find symbol
symbol: variable DataKeys
location: class glslplugin.actions.GLSLDeduceExpressionTypeAction
Error:(120, 41) java: cannot find symbol
symbol: variable DataKeys
location: class glslplugin.actions.GLSLDeduceExpressionTypeAction
Error:(129, 26) java: cannot find symbol
symbol: variable PsiUtil
location: class glslplugin.actions.GLSLDeduceExpressionTypeAction
glsl4idea/src/glslplugin/structureview/GLSLStructureViewFactory.java
Error:(34, 52) java: is not abstract and does not override abstract method createStructureViewModel(com.intellij.openapi.editor.Editor) in com.intellij.ide.structureView.TreeBasedStructureViewBuilder
I'll see if I can fix some of those, hopefully without breaking the plugin in IDEA.