Use proper cmake detection of features.

This attempts to organize all needed items and places appropriate defines
in a single location. Redundant comparisons/defines were removed when noticed.

A few caveats, however:

* This may be better targeted towards the 5_1_0 branch instead of master right now.
* Cmake will run a little slower on configuration/generation now. That's due to the sanity checks it runs.
* There are some more checks to be added later, but this should be a solid start.

Thanks to the [libical](https://github.com/libical/libical) project for inspiration.

src/archutils/Unix/Backtrace.cpp

@@ -2,13 +2,17 @@
 #include "Backtrace.h"
 #include "RageUtil.h"
 
+#if defined(HAVE_UNISTD_H)
 #include <unistd.h>
+#endif
 #include <cstdlib>
 #include <sys/types.h>
 #include <sys/wait.h>
 #include <sys/stat.h>
 #include <cerrno>
+#if defined(HAVE_FCNTL_H)
 #include <fcntl.h>
+#endif
 
 #if defined(BACKTRACE_METHOD_X86_LINUX)
 #include "archutils/Common/PthreadHelpers.h"