Add modifications to support cross-platform builds

[freakboy3742]

Adds four changes to support cross-platform builds:

1. Adds support for applying patches to downloaded source. If PATCH_DIR is defined, and contains a packagename-version.patch file that matches the package being unpacked, that patch will be applied to the source tree as part of the fetch_unpack step. This is required because some libraries haven't accepted Android and iOS compliance patches (most notably, autotools itself, which has repeatedly ignored/ghosted requests to add support for differentiating iOS simulators from iOS devices).
2. Adds support for a HOST_CONFIGURE_FLAGS option that is a blanket "include these flags in the invocation of ./configure" option.
3. Adds support for a HOST_CMAKE_FLAGS option. The analog of (2), but for cmake.
4. Disables the install of cmake if it's already installed.

I've been able to use these options, plus some small patches, to build Pillow's dependencies for iOS - e.g.:

HOST_CONFIGURE_FLAGS="--disable-shared --enable-static --host=arm64-apple-ios --build=arm64-apple-darwin"
HOST_CMAKE_FLAGS="-DCMAKE_SYSTEM_NAME=iOS -DCMAKE_SYSTEM_PROCESSOR=aarch64 -DCMAKE_OSX_DEPLOYMENT_TARGET=13.0 -DCMAKE_OSX_SYSROOT=<sdk path> -DBUILD_SHARED_LIBS=NO"

I've added a test for the patching option; but I haven't added any new tests for the HOST_CONFIGURE_CFLAGS and HOST_CMAKE_FLAGS options - partly because they're mostly innocuous changes; but mostly because I'm not sure what testing strategy would make sense here. Adding a full iOS build pass seems excessive; but I can't think of an obvious alternative for testing these new capabilities.

[mattip]

Seems reasonable. Thanks @freakboy3742