core: Fix --disable-log option to remove logging functionality

core: Fix --disable-log option to remove logging functionality

Prior to this commit, building the library with the '--disable-log'
option would compile out the debug messages, but the logging functions
remained and would never be used. This commit brings the logging
functions themselves into consideration when building the library.

Additionally, the logging code has been optimized to compile away
unnecessary checks when the '--enable-debug-log' is used and to remove
the debug variables from the context if the library is compiled in such
a way that the debug level is not configurable.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>