GCovGcdaGenerator breaks some builds

[ptomulik]

GCovGcdaGenerator works by searching children of a given node while parsing SConscript files. This breaks some builds. Most often the effect is that .sconsign file is created in wrong place when using hierarchical builds and SCons can't find it when finishing. The issue is described in this thread: http://scons.tigris.org/ds/viewMessage.do?dsForumId=1272&dsMessageId=2413891 but there is no solution. Using children() while parsing SConscript is an evil and we have to find better way to implement same functionality.

Workaround: use env.SconsignFile() with absolute path at the beginning of your SConstruct.