build-sys: use backticks rather than $() for commands in configure

build-sys: use backticks rather than $() for commands in configure

The configure had both backticks and $() in use.  These command
susbstitutions are mutually interchangeable, so one should pick only one
form.  In this case backticks were favored because they are known with
greater range of shells, making the backticks to be more portable.

References: https://www.gnu.org/software/autoconf/manual/autoconf-2.67/html_node/Shell-Substitutions.html#index-g_t_0024_0028_0040var_007bcommands_007d_0029-1632
Signed-off-by: Sami Kerola <kerolasa@iki.fi>