build: run configure under bash (autoconf 2.73 backtick quoting breaks dash)

Run #472 (post -W no-error fix) got past autoreconf but failed in ./configure:

  ./configure: line 11244: syntax error near unexpected token
    `as_ac_var=`printf '%s\n' "ac_cv_func_$ac_func" | sed "$as_sed_sh"``

autoconf 2.73's generated configure uses backtick command substitution
inside variable assignments with nested quoting. dash/MSYS2's /bin/sh
parses this as a syntax error because the inner backticks don't nest
cleanly inside the outer backtick expression.

Force CONFIG_SHELL=bash and invoke configure via "$CONFIG_SHELL"
so the generated script is parsed by bash regardless of platform
(MSYS2 MINGW64 defaults to dash for /bin/sh, which is what bit us).

src/tor/build-libtor.sh

@@ -17,7 +17,12 @@ if [[ ! -x "./configure" ]] || [[ "${AUTORECONF_FORCE:-0}" == "1" ]]; then
 fi
 
 echo "Configuring Tor static library build from: $TOR_SRC_DIR"
-./configure \
+# autoconf 2.73's generated ./configure uses backtick command substitution
+# inside variable assignments (e.g. `as_ac_var=\`printf ... | sed ...\``)
+# which dash/MSYS2's /bin/sh rejects with a syntax error. Force bash
+# explicitly so the same script works on Windows MSYS2 and macOS/Linux.
+export CONFIG_SHELL="${CONFIG_SHELL:-$(command -v bash)}"
+"$CONFIG_SHELL" ./configure \
   --enable-static-tor \
   --disable-module-relay \
   --disable-module-dirauth \