make other backends error if you use C externs

Author: MESYETI

source/backends/linux86.d

@@ -357,6 +357,8 @@ class BackendLinux86 : CompilerBackend {
 						);
 					}
 					output ~= format("sub r15, %d\n", word.params.length * 8);
+
+					// TODO: support more than 6 parameters
 
 					output ~= format("call %s\n", node.name);
 

source/backends/rm86.d

@@ -696,6 +696,10 @@ class BackendRM86 : CompilerBackend {
 	}
 
 	override void CompileExtern(ExternNode node) {
+		if (node.externType == ExternType.C) {
+			Error(node.error, "This backend doesn't support C externs");
+		}
+
 		Word word;
 		word.raw         = node.externType == ExternType.Raw;
 		words[node.func] = word;

source/backends/uxn.d

@@ -663,6 +663,11 @@ class BackendUXN : CompilerBackend {
 	}
 
 	override void CompileExtern(ExternNode node) {
+		if (node.externType == ExternType.C) {
+			Error(node.error, "This backend doesn't support C externs");
+		}
+
+
 		Word word;
 		word.raw         = node.externType == ExternType.Raw;
 		words[node.func] = word;