Maintenenace: fix several compiler warnings in C code

Maintenenace: fix several compiler warnings in C code

Due to incorrect handling of C source files, the typical compiler warning
settings had not been applied to C code, but instead only to C++ code.

After fixing the build system setting, this patch fixed most of the
compiler warnings in C code.

other fixes:
- GLXApplicationExample: fixed compiler warnings
- CMake: fixed several compiler warnings
- LayerManagerBase: fix compiler warning
- LayerManagerUtils: suppress DLT macro related warnings, if DLT is enabled
- TextureBinders: fixed compiler warning

Signed-off-by: Timo Lotterbach <timo.lotterbach@bmw-carit.de>