Recent addition of the GCC __attribute__(alloc_size) to some functions
in mem.h broke the build when using Clang 3.5. This commit introduces
configure-time checks for all GCC attributes used in the Serval source
code, and adapts accordingly.
Original DEBUG() and DEBUGF() macros renamed to _DEBUG() and _DEBUGF()
New DEBUG() and DEBUGF() macros, first argument is flagname
New DEBUGF2(foo, bar, ...) macro does if(config.debug.foo||config.debug.bar) test
Replace almost all config.debug.xxx references to IF_DEBUG(xxx)
