glsl: Only warn for macro names containing __
glsl: Only warn for macro names containing __

From page 14 (page 20 of the PDF) of the GLSL 1.10 spec:

    "In addition, all identifiers containing two consecutive underscores
     (__) are reserved as possible future keywords."

The intention is that names containing __ are reserved for internal use
by the implementation, and names prefixed with GL_ are reserved for use
by Khronos.  Names simply containing __ are dangerous to use, but should
be allowed.

Per the Khronos bug mentioned below, a future version of the GLSL
specification will clarify this.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Cc: "9.2 10.0 10.1" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Tested-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Anuj Phogat <e2026475996c48f0b97fd37e84129b3912980428@gmail.com>
Tested-by: Darius Spitznagel <b7ed27f881898dc6a35379ef9cce4011ffb66934@goodbytez.de>
Cc: Tapani Pälli <6405c7ed06b93e0a5000583c1b7918e78875facb@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71870
Bugzilla: Khronos #11702

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