[AltiVec] include altivec.h only when the instructions are available (#315)

sleef.h will use `vector ...` types unprotected on POWER, which
requires including AltiVec just like, for example NEON requires
arm_neon.h.  However, unlike NEON, SVE, and SSE, until this patch
SLEEF required including altivec.h prior to including sleef.h.

Co-authored-by: Naoki Shibata <shibatch@users.sourceforge.net>