icecc.bbclass: replace deprecated bash command substitution

icecc.bbclass: replace deprecated bash command substitution

- build some packages with icecc enabled is not supported
  because of the folling that disables the icecc:

  DEBUG: while parsing set_icecc_env, unable to handle non-literal command '$ICECC_CC'

- it can be replicated with:

 bitbake make && bitbake make -c cleansstate && bitbake make -DD
 grep ICECC_CC tmp/log/cooker/qemux86-64/console-latest.log

- bash command substitution backquote deprecated

 https://www.gnu.org/software/bash/manual/html_node/Command-Substitution.html
 https://mywiki.wooledge.org/BashFAQ/082

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>