Windows build fixes

commit 48d897f6:

nvenc: Fix build when cuda is found but nvenc is disabled
tests\check\meson.build:21:5: ERROR:  Unknown variable "use_nvenc_gl".

Also do the same change in nvdec, just to be defensive about future
issues.

commit 4430fa0e:

closedcaption: Fix build with Visual Studio
../ext/closedcaption/io-sim.c(440): error C2065: 'M_PI': undeclared identifier
../ext/closedcaption/io-sim.c(442): error C2065: 'M_LN2': undeclared identifier