demos: improve nuklear integrationo

1. Warn if submodule is missing
2. Link against -lm, fixes acos not resolving
12 jobs for master in 7 minutes and 25 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Compile
passed aarch64 #597289
docker aarch64

00:00:09

passed linux #597285
docker amd64

00:00:12

passed macos #597290
amd64 catalina

00:00:08

passed scan #597291
docker amd64

00:00:26

passed static #597286
docker amd64

00:00:11

passed win32 #597287
docker amd64

00:00:09

passed win64 #597288
docker amd64

00:00:12

 
  Test
passed gpu #597293
gpu

00:01:45

78.8285%
passed llvmpipe #597292
docker amd64

00:00:34

 
  Sanitize
passed asan #597294
gpu

00:01:59

passed msan #597295
gpu

00:01:41

passed ubsan #597296
gpu

00:01:30