gcc-9: fix CMake parsing of CPU architecture

GCC-9 adds known options section for march that causes regex
replace failure in CMake file

Fixes github issue intel#136

CMakeLists.txt

@@ -191,6 +191,8 @@ else()
         set (EXEC_ARGS ${CC_ARG1} -c -Q --help=target -march=native -mtune=native)
         execute_process(COMMAND ${CMAKE_C_COMPILER} ${EXEC_ARGS}
             OUTPUT_VARIABLE _GCC_OUTPUT)
+        string(FIND "${_GCC_OUTPUT}" "Known" POS)
+        string(SUBSTRING "${_GCC_OUTPUT}" 0 ${POS} _GCC_OUTPUT)
         string(REGEX REPLACE ".*march=[ \t]*([^ \n]*)[ \n].*" "\\1"
             GNUCC_ARCH "${_GCC_OUTPUT}")