build, refactor: Check that Homebrew's qt5 package is actually installed

This change unifies Homebrew packages workflow, and does not change
behavior.

configure.ac

@@ -642,16 +642,15 @@ 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).
 
-         qt5_prefix=$($BREW --prefix qt5 2>/dev/null)
          if $BREW list --versions berkeley-db4 >/dev/null && test "x$BDB_CFLAGS" = "x" && test "x$BDB_LIBS" = "x" && test "$use_bdb" != "no"; then
            bdb_prefix=$($BREW --prefix berkeley-db4 2>/dev/null)
            dnl This must precede the call to BITCOIN_FIND_BDB48 below.
            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