Various restructure of test framework handling of paths to avoid duplicate path editing which caused failed tests. Adjust many tests to deal with added subdirectories in build outputs now that toolset requirements are active.
lib png : z : <name>png ;
lib z : : <name>z ;
now works: if you link to 'png' you'll also link to 'z'.
lib png : z : <file>png.a ;
lib z : : <file>z.a ;
now works too. The 'prebuilt.jam' modules which used to handle <file> for
all target kinds is now removed.
[SVN r25703]