Merge bitcoin-core/secp256k1#1025: build: replace backtick command su…

…bstitution with $()

2b7c749 build: replace backtick command substitution with $() (fanquake)

Pull request description:

  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 anyways.

  Followup to bitcoin-core/secp256k1#1019 (review). I had thought there were more usages of this
  syntax, but seems like it's just the one.

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

  Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

ACKs for top commit:
  real-or-random:
    ACK 2b7c749
  hebasto:
    ACK 2b7c749, verified that this is the only case.

Tree-SHA512: 6192f5efe437ff428ce7843ac595049a1aa7969a9e696f649cfd4820b28fc96ad0fabd6eec0ec1ca404763f02e64af6a99e57666a00d8749c6212a0646211991

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