0031070: Configuration - fix building issues when using Emscripten toolchain

Handled __EMSCRIPTEN__ macros to:
- Workaround atomics (__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 is undefined, but GCC atomics are provided).
- Suppress non-standard header <sys/signal.h> warning.
- Return OSD_LinuxREDHAT.
- Avoid inclusion of XLib headers.
- Skip fontconfig library.
- Enable EGL+GLES path (translated by Emscripten into WebGL).
- Skip eglCreatePbufferSurface() not implemented by Emscripten EGL.

Fixed Graphic3d_Vec4.hxx usage within Quantity_ColorRGBA.hxx.

OpenGl_ShaderManager::defaultGlslVersion() now prefers GLSL 300 es when WebGL 2.0 is available,
as there no any OpenGL ES greater than 3.0 emulation so far.

Shaders_Declarations.glsl - added workaround for GLSL compilation on WebGL 1.0
by defining Light properties accessors as macros instead of functions
('[]' : Index expression must be constant).

OpenGl_FrameBuffer::Init() - added workaround for initialization of GL_DEPTH24_STENCIL8
depth-stencil attachment on WebGL 1.0 + GL_WEBGL_depth_texture extension.

OpenGl_Context::Vec4FromQuantityColor() now considers myIsSRgbActive flag
to handle use case, when Immediate Layer is drawn directly into window buffer,
which is not sRGB-ready.

Added new sample - OCCT WebGL viewer.

src/Standard/Standard_Atomic.hxx

@@ -44,7 +44,7 @@ inline int Standard_Atomic_Decrement (volatile int* theValue);
 inline bool Standard_Atomic_CompareAndSwap (volatile int* theValue, int theOldValue, int theNewValue);
 
 // Platform-dependent implementation
-#if defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
+#if defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4) || defined(__EMSCRIPTEN__)
 // gcc explicitly defines the macros __GCC_HAVE_SYNC_COMPARE_AND_SWAP_*
 // starting with version 4.4+, although built-in functions
 // are available since 4.1.x. However unless __GCC_HAVE_SYNC_COMPARE_AND_SWAP_*