Fix compilation warnings and re-enable werror=yes on Travis

Fix -Wunused-variable, -Wunused-but-set-variable and -Wswitch warnings
raised by GCC 8 and 9.

Fix -Wunused-function, -Wunused-private-field and
-Wtautological-constant-out-of-range-compare raised by Clang.

Fix MSVC 2019 warning C4804 (unsafe use of type 'bool' in comparison
operation).

GCC -Wcpp warnings/Clang -W#warnings (`#warning`) are no longer raising
errors and will thus not abort compilation with `werror=yes`.

Treat glslang headers are system headers to avoid raising warnings.

Re-enables us to build with `werror=yes` on Linux and macOS, thus
catching warnings that would be introduced by new code.

Fixes #36132.

modules/glslang/SCsub

@@ -58,7 +58,11 @@ if env['builtin_glslang']:
 
     thirdparty_sources = [thirdparty_dir + file for file in thirdparty_sources]
 
-    env_glslang.Prepend(CPPPATH=[thirdparty_dir])
+    # Treat glslang headers as system headers to avoid raising warnings. Not supported on MSVC.
+    if not env.msvc:
+        env_glslang.Append(CPPFLAGS=['-isystem', Dir(thirdparty_dir).path])
+    else:
+        env_glslang.Prepend(CPPPATH=[thirdparty_dir])
 
     env_thirdparty = env_glslang.Clone()
     env_thirdparty.disable_warnings()