cpu.cmake: improve webp_check_compiler_flag output
cpu.cmake: improve webp_check_compiler_flag output

WEBP_HAVE_FLAG_LOCAL -> WEBP_HAVE_FLAG_${WEBP_SIMD_FLAG} this will
include the flag being tested in the output:
-- Performing Test WEBP_HAVE_FLAG_NEON

Change-Id: I1c0a143a857b16e4eb1fcf8b23c176380a5fef29

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp