Need reviewer: Retire ALT_MSDEVTOOLS_PATH (Windows only)

[Kelly O'Hair]

Need reviewer: Retire ALT_MSDEVTOOLS_PATH (Windows only)

The use of ALT_MSDEVTOOLS_PATH with VS2010 has become so confused that  
the simplest solution
is to remove it completely and rely on COMPILER_PATH when building  
with VS2010, the expected compiler.

Some minor restructuring and simplification of the Makefiles was  
needed to accommodate these changes.
At some point only VS2010 will be used for Windows builds as we become  
more and more dependent
on the newer Windows apis and options, so some of this is isolation of  
the VS2010 settings so that we
might completely remove the more complicated unneeded logic for the  
older compilers someday.
If VS2010 can be found on the system, it is assumed that VS2010 is  
being used, so use of older compilers
when the newer compiler is on the system would require setting  
VS2010_EXISTS=false, but there should
be little need for that. The logic in finding the VS2010 compiler  
location is as optimal as I could get it,
avoiding unnecessary exec's for nested makes.

One additional optimization was made to the FullPath define for CYGWIN  
builds to minimize exec's in
some situations

6360517: ALT_MSDEVTOOLS_PATH and rc.exe location, and rebase location
http://cr.openjdk.java.net/~ohair/openjdk7/jdk7-build-msdevtools/webrev/

-kro