include: Define macro to ignore -Wdeprecated-declarations on GCC

"-Wdeprecated-declarations" warning option can be helpful to track
deprecated function delarations but sometimes we need to disable this
warning for a good reason.

DIAGNOSTIC_IGNORE_DEPRECATED_DECLARATIONS is an existing macro but only
defined on Clang.  Since "-Wdeprecated-declarations" is also available on
GCC (>= 3.4.0), this commit adds equivalent definition as Clang.

__GNUC__ and __GNUC_MINOR__ are not checked because this header file seems
to assume GCC >= 4.6 (with "GCC diagnostic push/pop").

include/ChangeLog:

	* diagnostics.h (DIAGNOSTIC_IGNORE_DEPRECATED_DECLARATIONS):
	Define also on GCC.