Merge #1019: build: don't append valgrind CPPFLAGS if not installed (macOS)

214042a build: don't append valgrind CPPFLAGS if not installed (fanquake)

Pull request description:

  Valgrinds CPPFLAGS, i.e `-I/usr/local/opt/valgrind/include`, are currently added to CPPFLAGS, regardless of whether valgrind is installed. This changes configure so that they are only added if valgrind is available. i.e the output of `brew list --versions valgrind` is non-null.

ACKs for top commit:
  real-or-random:
    ACK 214042a
  hebasto:
    ACK 214042a, tested on macOS Big Sur 11.6.1 (20G224, Intel).

Tree-SHA512: 5101636a0a12f1941b01967ca8eab7aa20f44db0d1ef4571a5ad6026bb89494b983465d34d93c8b17a260b695116792991da53d135bc19a3c9e974f5266a90af

Author: real-or-random

configure.ac

@@ -43,8 +43,8 @@ case $host_os in
          # These Homebrew packages may be keg-only, meaning that they won't be found
          # in expected paths because they may conflict with system files. Ask
          # Homebrew where each one is located, then adjust paths accordingly.
-         valgrind_prefix=`$BREW --prefix valgrind 2>/dev/null`
-         if test x$valgrind_prefix != x; then
+         if $BREW list --versions valgrind >/dev/null; then
+           valgrind_prefix=`$BREW --prefix valgrind 2>/dev/null`
            VALGRIND_CPPFLAGS="-I$valgrind_prefix/include"
          fi
        else