Gradle File Preprocessor

build.gradle

ext {
    skipFunc = true
    skipPrint = true
}

task preprocessor(type: Copy) {
    boolean inTag = false
    from 'src/main/kotlin'
    into "$buildDir/processed-src"
    filter { line ->
        String updated = line

        try {
            if (line.trim().startsWith("//PP.IF") && project.ext[line.replace("//PP.IF", "").trim()] as boolean)
                inTag = true
        } catch (MissingPropertyException ignored) { }

        if (inTag) updated = null

        if (line.trim().startsWith("//PP.ENDIF"))
            inTag = false

        return updated
    }
}

preprocessor.dependsOn(clean)

['compileKotlin'].each {
    tasks[it].doFirst {sourceSets.main.kotlin.srcDirs = ["$buildDir/processed-src"]}
    tasks[it].dependsOn(preprocessor)
    tasks[it].doLast {sourceSets.main.kotlin.srcDirs = ["src/main/kotlin"]}
}

Processed.kt

package example

fun main() {
    //PP.IF yes
    println("This should print")
    //PP.ENDIF
  
    //PP.IF skipPrint
    println("This should not.")
    //PP.ENDIF
  
    //PP.IF skipFunc
    func()
    //PP.ENDIF
}

//PP.IF skipFunc
fun func() {
    println("I am a print from a function!")
}
//PP.ENDIF
//PP.ENDIF

This allows configuration in the Gradle file to skip parts of the code. Note: The preprocessor task relies on clean which results in increased build time for larger projects. A better solution is something to look into in the future.