Fixed finding CURL on systems with multiarch support

Fixed finding CURL on systems with multiarch support

fixes https://bugs.php.net/bug.php?id=74125

This commit makes the cURL config script aware of debian/ubuntu
[multiarch support][1] which installs architecture specific
headers in a different location.

It checks whether the `dpkg-architecture` script exists and is
executeable, if that is the case, the multiarch architecture is
detected by calling `dpkg-architecture -qDEB_HOST_MULTIARCH` as
documented in [debian multiarch implementation docs][2]:

> `/usr/include/<triplet>`: used for arch-varying headers

[1]: https://wiki.debian.org/Multiarch
[2]: https://wiki.debian.org/Multiarch/Implementation