scons: Specify C99 throughout all the tree.

scons: Specify C99 throughout all the tree.

MSVC may not support full C99, but supports more than plain C90. And
-pedantic without -std=c99 generates too many spurious warnings
(specially C++ style comments) to be of any use.

Note that using certain C99 features in the cross-platform parts of Gallium
is still not possible; namely mid-of-scope variable declarations and named
structure initializers will break MSVC builds.