Add CHANGES note for GL #525.

5031.   [cleanup]       Various defines in platform.h has been either dropped
                        if always or never triggered on supported platforms
                        or replaced with config.h equivalents if the defines
                        didn't have any impact on public headers.  Workarounds
                        for LinuxThreads have been removed because NPTL is
                        available since Linux kernel 2.6.0.

CHANGES

+5031.	[cleanup]	Various defines in platform.h has been either dropped
+			if always or never triggered on supported platforms
+			or replaced with config.h equivalents if the defines
+			didn't have any impact on public headers.  Workarounds
+			for LinuxThreads have been removed because NPTL is
+			available since Linux kernel 2.6.0.  [GL #525]
+
 5030.	[bug]		Align CMSG buffers to a 64-bit boundary, fixes crash
 			on architectures with strict alignment. [GL #521]