Add space to play nicely with --with-cppflags parameters

[stanhu]

Attempting to build charlock_holmes with --with-cppflags will fail unless you append a space at the end. For example, to build with XCode 16.0 and macOS SDK 15.0, for some reason I had to manually add the C++ include path with a trailing space:

gem install charlock_holmes -- --enable-static --with-cppflags='-I/Library/Developer/CommandLineTools/SDKs/MacOSX15.0.sdk/usr/include/c++/v1 '