curl-override.m4: provide AC_CONFIG_MACRO_DIR definition conditionally

Provide a 'traceable' AC_CONFIG_MACRO_DIR definition only when using
an autoconf version that does not provide it, instead of what we were
doing up to now of providing and overriding AC_CONFIG_MACRO_DIR for
all autoconf versions.

m4/curl-override.m4

@@ -2,7 +2,7 @@
 #***************************************************************************
 
 # File version for 'aclocal' use. Keep it a single number.
-# serial 5
+# serial 6
 
 dnl CURL_OVERRIDE_AUTOCONF
 dnl -------------------------------------------------
@@ -89,12 +89,12 @@ m4_defun([_LT_AC_LIBTOOL_SYS_PATH_SEPARATOR],
 [CURL_CHECK_PATH_SEPARATOR
 m4_define([$0],[])])
 
-dnl Override Autoconf's AC_CONFIG_MACRO_DIR (DIR)
+dnl Provide Autoconf's AC_CONFIG_MACRO_DIR (DIR)
 dnl -------------------------------------------------
-dnl This is an emulation of Autoconf's 2.61 macro.
-dnl This is done to use fixed macro across Autoconf
-dnl versions, and avoid warnings from modern libtool
-dnl which traces usage of this macro.
+dnl Allow usage of mentioned macro in configure.ac
+dnl even with ancient Autoconf versions, such as 2.57,
+dnl that do not provide a definition for this macro.
 
-AC_DEFUN([AC_CONFIG_MACRO_DIR],[:])
+m4_ifndef([AC_CONFIG_MACRO_DIR],
+[AC_DEFUN([AC_CONFIG_MACRO_DIR],[:])])