Guile 2.2 calls `__sincos`, which should be defined in `math.h`, but the
version of that header in the 10.12 SDK is very old (predating 10.12).
Until there is a version of that header available, use the 11.0 SDK to
build Guile.
The include/ directory for libunistring is inside the libunistring.dev
package, not libunistring, thus the .pc file was pointing to a
non-existing path.
