build: Fix wrong CMake CFLAGS variable
build: Fix wrong CMake CFLAGS variable

Use CMAKE_C_FLAGS to actually make CFLAGS defined for the build. Also
turn on GNU extensions as libuv requires them.

Signed-off-by: Pekka Enberg <add4fcd06328a394f0ad91feda7ee057316dc5ed@iki.fi>

jvirtanen/helix,jvirtanen/helix,penberg/helix,penberg/helix