glsl: simplify the M_PI*f macros, fixes build on OpenBSD
glsl: simplify the M_PI*f macros, fixes build on OpenBSD

The M_PI*f macros used a preprocessor paste to append 'f'
to M_PI defines, which works if the values are only numbers
but breaks on OpenBSD where M_PI definitions have casts
and brackets to meet requirements of a future version of POSIX,

http://austingroupbugs.net/view.php?id=801
http://austingroupbugs.net/view.php?id=828

Simplify the M_PI*f macros by using casts directly in the defines
as suggested by Kenneth Graunke.

Cc: "10.2" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78665
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Signed-off-by: Jonathan Gray <666d82badf37dc240bacece830d7f52411b6fee3@jsg.id.au>

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