PP: don't give errors on some tokens under #if 0 (or similar).

Fixes #1295.

Tokens that are accepted by any version of HLSL or GLSL should be
allowed when #ifdef'd off, such that errors are not reported.
diff --git a/gtests/AST.FromFile.cpp b/gtests/AST.FromFile.cpp
index fea7586..061c86c 100644
--- a/gtests/AST.FromFile.cpp
+++ b/gtests/AST.FromFile.cpp
@@ -99,6 +99,7 @@
         "cppComplexExpr.vert",
         "cppDeepNest.frag",
         "cppPassMacroName.frag",
+        "cppRelaxSkipTokensErrors.vert",
         "badChars.frag",
         "pointCoord.frag",
         "array.frag",