cmpc: when infinity is used, emit macro "INFINITY"
and "-INFINITY" for negative infinity. These corner-cases lead to compilation error, so no regression will be imposed even against non-c99 compilers. INFINITY and NAN macros are introduced by C99 standard, so such code requires C-backend to support this math extension. Fixes #156. Signed-off-by: Daniel Kochmański <email@example.com>