RFR: JDK-8071329: Stop exporting INCLUDE and LIB when building on windows

[Erik Joelsson]

Hello,

Please review this patch, which makes it possible to take a compile 
command line from the make debug log on Windows, and rerun it in a 
normal cygwin environment, without the need for running vsvars*.bat first.

When building native code on windows, using Visual Studio, configure 
extracts the build environment from the setup .bat file provided by VS 
and sets 3 variables in spec.gmk: PATH, INCLUDE and LIB. These 3 
variables are also exported into the environment in spec.gmk, so that 
every tool run by the build will see them. While this is convenient, it 
makes the command lines used by the build unusable outside of the build, 
unless you also export these variables with the correct values.

I have removed the need for INCLUDE and LIB to be exported, by 
converting their contents into compiler and linker flags. These flags 
conceptually fit well in the recent SYSROOT_CFLAGS and SYSROOT_LDFLAGS 
variables.

The PATH variable would be nice to not have to set, and while not 
setting it seems to work most of the time, I suspect that there are 
cases when it won't work. More specifically, in certain environments, 
some dll needed by the compiler program might not be on the path without 
it. So I left it being set for now.

The new LDFLAGS requires unpack200.exe to stop being linked differently 
to all other executables. There is no reason for this discrepancy that I 
can find, it just seems like someone did a bit of a quick hack getting 
it to build long ago in the old build, and we wanted to keep it 
equivalent in build-infra.

The hotspot build still requires the variables to be exported, so they 
are still being defined in hotspot-spec.gmk.

While working on this, I stumbled on a problem when running "make 
reconfigure". The PATH variable value, since exported in make, would get 
longer and longer for each time you run reconfigure. I fixed this by 
saving the original path and resetting it before running configure from 
make.

Bug: https://bugs.openjdk.java.net/browse/JDK-8071329
Webrevs:
http://cr.openjdk.java.net/~erikj/8071329/webrev.root.01/
http://cr.openjdk.java.net/~erikj/8071329/webrev.jdk.01/

/Erik