message: build, refactor: Check that Homebrew's qt5 package is actually installed
  This change unifies Homebrew packages workflow, and does not change
  behavior.
diff: @@ -643,7 +643,6 @@ case $host in
  dnl It's safe to add these paths even if the functionality is disabled by
  dnl the user (--without-wallet or --without-gui for example).
  openssl_prefix=`$BREW --prefix openssl 2>/dev/null`
  - qt5_prefix=$($BREW --prefix qt5 2>/dev/null)
  if test x$openssl_prefix != x; then
  PKG_CONFIG_PATH="$openssl_prefix/lib/pkgconfig:$PKG_CONFIG_PATH"
  export PKG_CONFIG_PATH
  @@ -654,9 +653,9 @@ case $host in
  BDB_CFLAGS="-I$bdb_prefix/include"
  BDB_LIBS="-L$bdb_prefix/lib -ldb_cxx-4.8"
  fi
  - if test x$qt5_prefix != x; then
  - PKG_CONFIG_PATH="$qt5_prefix/lib/pkgconfig:$PKG_CONFIG_PATH"
  - export PKG_CONFIG_PATH
  +
  + if $BREW list --versions qt5 >/dev/null; then
  + export PKG_CONFIG_PATH="$($BREW --prefix qt5 2>/dev/null)/lib/pkgconfig:$PKG_CONFIG_PATH"
  fi
  fi
  else
  