build: replace backtick command substitution with $()

This is only needed for the very oldest of non-POSIX-compatible shells.
Note that this code will also only be executed on macOS, where it'd be
very unlikely to run into such a shell.

Followup to bitcoin#1019.

See:
https://github.com/koalaman/shellcheck/wiki/SC2006

Co-authored-by: Hennadii Stepanov <[email protected]>

Author: fanquake

configure.ac

@@ -44,7 +44,7 @@ case $host_os in
          # in expected paths because they may conflict with system files. Ask
          # Homebrew where each one is located, then adjust paths accordingly.
          if $BREW list --versions valgrind >/dev/null; then
-           valgrind_prefix=`$BREW --prefix valgrind 2>/dev/null`
+           valgrind_prefix=$($BREW --prefix valgrind 2>/dev/null)
            VALGRIND_CPPFLAGS="-I$valgrind_prefix/include"
          fi
        else