git-compat-util: do not step on MAC_OS_X_VERSION_MIN_REQUIRED

MAC_OS_X_VERSION_MIN_REQUIRED may be defined by the builder to a specific version in order to produce compatible binaries for a particular system. Blindly defining it to MAC_OS_X_VERSION_10_6 is bad. Additionally MAC_OS_X_VERSION_10_6 will not be defined on older systems and should AvailabilityMacros.h be included on such as system an error will result. However, using the explicit value of 1060 (which is what MAC_OS_X_VERSION_10_6 is defined to) does not solve the problem. The changes that introduced stepping on MAC_OS_X_VERSION_MIN were made in b195aa00 (git-compat-util: suppress unavoidable Apple-specific deprecation warnings) to avoid deprecation warnings. Instead of blindly setting MAC_OS_X_VERSION_MIN to 1060 change the definition of DEPRECATED_ATTRIBUTE to empty to avoid the warnings. This preserves any MAC_OS_X_VERSION_MIN_REQUIRED setting while avoiding the warnings as intended by b195aa00. Signed-off-by: Kyle J. McKay <mackyle@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>