change build dir to ig-c99

generators should put their artifacts in a dir named after themselves, here ig-c99

lm-c99/ig-c99.ysl2

@@ -149,8 +149,7 @@ tstylesheet {
     };
 
 
-
-    template "idl" document "idl_core.h", "text" {
+    template "idl" document "ig-c99/idl_core.h", "text" {
         ||
         #pragma once
 
@@ -169,7 +168,7 @@ tstylesheet {
         ||
     }
 
-    template "api" document "{@name}_api.h", "text" {
+    template "api" document "ig-c99/{@name}_api.h", "text" {
         const "pkg_name", "../@name";
         const "pkg_version", "../version/text()";
         const "pkg_license", "../license/text()";
@@ -213,7 +212,7 @@ tstylesheet {
 
 
     // TYPE-TESTING
-    template "test-types" document "test_types.h", "text" {
+    template "test-types" document "ig-c99/test_types.h", "text" {
         ||
         // basetypes
         «func:typeinfo('string')»