scons SourceSignatures and TargetSignatures are deprecated

[Jan]

In newer scons versions, the env.SourceSignatures() and env.TargetSignatures() methods are deprecated which produces a warning:

gi27naj@nepomuk:/data/jan/clean/openwns-sdk$ ./playground.py install
Executing: scons dbg --warn=no-missing-sconscript -j 8 default
scons: Reading SConscript files ...

scons: warning: The env.SourceSignatures() method is deprecated;
 convert your build to use the env.Decider() method instead.
File "/data/jan/clean/openwns-sdk/SConstruct", line 11, in <module>

scons: warning: The env.TargetSignatures() method is deprecated;
 convert your build to use the env.Decider() method instead.
File "/data/jan/clean/openwns-sdk/SConstruct", line 12, in <module>

The 2 methods can be replaced easily by the new Decider() method. The straight-forward way to replace this would be to use Decider('timestamp-match') but I think the combination of a time stamp and a MD5 checksum is even more elegant, see the diff below:

-SourceSignatures('timestamp')
-TargetSignatures('timestamp')
+Decider('MD5-timestamp')

As these are only 2 lines, I do not push a whole new branch for this.