mk: fix shell errors when building with clang

mk: fix shell errors when building with clang

On my system, the version of the compiler is not properly retrieved,
resulting in strange logs when building the dpdk:
  /bin/sh: line 0: test: too many arguments

This happens when mk/toolchain/clang/rte.toolchain-compat.mk is included
from a directory that use gcc to build (ex: kernel modules). In that
case, the CLANG_VERSION variable contains spaces that breaks some shell
calls to the test program.

The error is because the output of "gcc -v" on my system contains 2 lines
that matches the "version" string:

  Configured with: ../src/configure -v \
    --with-pkgversion='Debian 6.3.0-6' [...]
  gcc version 6.3.0 20170205 (Debian 6.3.0-6)

This may be specific to Debian. Fix it by specializing the grep.

Fixes: 2ef6eea891e5 ("mk: add clang toolchain")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>