Rewrite the Compiler detection tests for deprecation and unused. Simplify and make more safe, clang or compilers supporting __has_extension() and __has_attribute() should work perfectly. There are now GCC version tests for all these features, however there seems to be some disagreement online about which GCC versions introduced these features, so I picked the versions that I felt had the highest consensus. Can revisit the specific versions if new information is available: unused attribute: GCC 3.1 (BSD seems to think 2.7) deprecated attribute: GCC 3.2 (Hedley seems to think 4.0) deprecated with message attribute: GCC 4.5

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=3
DW_MINOR_VERSION=1