configure: use AC_CHECK_TOOL for objcopy, strings and gperf

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