Fixes for native client/PPAPI Linux

1) A few places assumed REGAL_SYS_PPAPI meant Windows/MSVC when in fact
Pepper is a valid option on Mac and Linux as well.

2) Initialize maxVaryings to zero before calling GetIntegerv. Pepper
GLES GetIntegerv for better or worse asserts that the output destination
memory is initialized to zero or negative one, though I saw nothing in
the spec that indicates it is required.