patch 8.0.1338: USE_IM_CONTROL is confusing and incomplete

Problem:    USE_IM_CONTROL is confusing and incomplete.
Solution:   Just use FEAT_MBYTE.  Call 'imactivatefunc' also without GUI.
diff --git a/src/option.h b/src/option.h
index 57126a3..c9fa4d3 100644
--- a/src/option.h
+++ b/src/option.h
@@ -585,11 +585,11 @@
 #define IM_OVER_THE_SPOT	1L
 EXTERN long	p_imst;		/* 'imstyle' */
 #endif
-#if defined(FEAT_EVAL) && defined(USE_IM_CONTROL)
+#if defined(FEAT_EVAL) && defined(FEAT_MBYTE)
 EXTERN char_u	*p_imaf;	/* 'imactivatefunc' */
 EXTERN char_u	*p_imsf;	/* 'imstatusfunc' */
 #endif
-#ifdef USE_IM_CONTROL
+#ifdef FEAT_MBYTE
 EXTERN int	p_imcmdline;	/* 'imcmdline' */
 EXTERN int	p_imdisable;	/* 'imdisable' */
 #endif