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