MINOR: build: discard echoing in help target

MINOR: build: discard echoing in help target

When V=1 is used in conjuction with help, the output becomes pretty
difficult to read properly.

  $ make TARGET=linux-glibc V=1 help
  ..
    DEBUG_USE_ABORT: use abort() for program termination, see include/haproxy/bug.h for details
  echo; \
     if [ -n "" ]; then \
       if [ -n "" ]; then \
          echo "Current TARGET: "; \
       else \
          echo "Current TARGET:  (custom target)"; \
       fi; \
     else \
       echo "TARGET not set, you may pass 'TARGET=xxx' to set one among :";\
       echo "  linux-glibc, linux-glibc-legacy, solaris, freebsd, dragonfly, netbsd,"; \
       echo "  osx, openbsd, aix51, aix52, aix72-gcc, cygwin, haiku, generic,"; \
       echo "  custom"; \
     fi

  TARGET not set, you may pass 'TARGET=xxx' to set one among :
    linux-glibc, linux-glibc-legacy, solaris, freebsd, dragonfly, netbsd,
    osx, openbsd, aix51, aix52, aix72-gcc, cygwin, haiku, generic,
    custom
  echo;echo "Enabled features for TARGET '' (disable with 'USE_xxx=') :"

  Enabled features for TARGET '' (disable with 'USE_xxx=') :
  set --        POLL                                  ; echo "  $*" | (fmt || cat) 2>/dev/null
    POLL
  echo;echo "Disabled features for TARGET '' (enable with 'USE_xxx=1') :"

  Disabled features for TARGET '' (enable with 'USE_xxx=1') :
  set -- EPOLL KQUEUE NETFILTER PCRE PCRE_JIT PCRE2 PCRE2_JIT  PRIVATE_CACHE THREAD PTHREAD_PSHARED BACKTRACE STATIC_PCRE STATIC_PCRE2 TPROXY LINUX_TPROXY LINUX_SPLICE LIBCRYPT CRYPT_H GETADDRINFO OPENSSL LUA FUTEX ACCEPT4 CLOSEFROM ZLIB SLZ CPU_AFFINITY TFO NS DL RT DEVICEATLAS 51DEGREES WURFL SYSTEMD OBSOLETE_LINKER PRCTL THREAD_DUMP EVPORTS OT QUIC; echo "  $*" | (fmt || cat) 2>/dev/null
    EPOLL KQUEUE NETFILTER PCRE PCRE_JIT PCRE2 PCRE2_JIT PRIVATE_CACHE

This commit ensure the help target always discard line echoing
regardless of V variable as done for reg-tests-help target.