CMake: fix webp_js compilation

Stick to the strict necessary for running webp_js,
and avoid building sub-lib or examples with heavy dependencies.

Change-Id: Ife4170a7839fb3201b2cf158d98d17bebe10008f
(cherry picked from commit 4cd0582d50eaa79140723c180fc6bfd3891b07f5)
1 file changed