configure: use AC_CHECK_TOOL for objcopy, strings and gperf

* using AC_PATH_TOOL does not allow to override it from shell environment
  which is useful when cross-compiling
* with external toolchain I have different HOST_PREFIX and HOST_SYS
  AC_PATH_TOOL is using HOST_SYS as prefix and fails to find objcopy
  which is available only as ${TARGET_PREFIX}-objcopy then it tries
  objcopy without prefix which is found on host, but that objcopy
  does not work for !host (e.g. arm when building on x86) libs

configure.ac

@@ -86,9 +86,9 @@ GOBJECT_INTROSPECTION_CHECK([1.31.1])
    AM_CONDITIONAL([HAVE_INTROSPECTION], [false])
    enable_introspection=no])
 
-AC_PATH_TOOL(OBJCOPY, objcopy)
-AC_PATH_TOOL(STRINGS, strings)
-AC_PATH_TOOL(GPERF, gperf)
+AC_CHECK_TOOL(OBJCOPY, objcopy)
+AC_CHECK_TOOL(STRINGS, strings)
+AC_CHECK_TOOL(GPERF, gperf)
 if test -z "$GPERF" ; then
         AC_MSG_ERROR([*** gperf not found])
 fi