euler: Add Euler → rotation matrix conversion

For completeness, similar to how we allow a quaternion → matrix
conversion.
matrix: Use Euler → matrix for rotation

Instead of converting a graphene_euler_t to a graphene_quaternion_t and
then extracting the rotation matrix.
build: Remove unnecessary file check

We only run autogen.sh from a Git clone, and each clone is guaranteed to
be complete; we don't need to check for the presence of a file (which
may or may not stay there) to ensure that we have a valid source
directory.