add m4/libcurl.m4

Arch Linux curl package stopped shipping libcurl.m4, stating needy
packages should include this on their own:
https://projects.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/curl&id=a5d620932b12d966ea7fa9e814590dd78562a12b

I am not sure if this is correct though... Could not find upstream hints by curl developer. So feel free to reject this - But does anybody know what's the right way?