SConstruct: split the functionality of the DEBUG option. DEBUG_BUILD (default: false...

SConstruct: split the functionality of the DEBUG option.  DEBUG_BUILD (default: false) enables '-Wall -g' during compiling.  DEBUG (default: true) includes code to support debug messages which are handy for many users when tracking down problems.  There is a performance penalty associated with DEBUG but at present the advantages probably outweigh the disadvantages.  The option name DEBUG was retained to avoid a need to propagate a name change throughout the code.

git-svn-id: svn+ssh://ffado.org/ffado/trunk@2507 2be59082-3212-0410-8809-b0798e1608f0