gitian: Reduce build time for boost windows dependency

Build only the mt-s (multithreaded, static runtime link)
variant of the library.

This cuts the build time in half.

Also prevent unpacked boost from ending up in output directory,
checking and hashing all the source files as output files this
takes a lot of time and is pointless.

contrib/gitian-descriptors/boost-win32.yml

@@ -16,7 +16,7 @@ files:
 - "boost-mingw-gas-cross-compile-2013-03-03.patch"
 script: |
   # Defines
-  INSTALLPREFIX="$OUTDIR/staging/boost"
+  INSTALLPREFIX="$HOME/install"
   HOST=i686-w64-mingw32
   # Input Integrity Check
   echo "fff00023dd79486d444c8e29922f4072e1d451fc5a4d2b6075852ead7f2b7b52  boost_1_55_0.tar.bz2" | shasum -c
@@ -57,10 +57,9 @@ script: |
   # Note: Might need these options in the future for 64bit builds.
   # "Please note that address-model=64 must be given to bjam command line on 64bit Windows for 64bit build; otherwise 32bit code will be generated."
   # "For cross-compiling the lib you must specify certain additional properties at bjam command line: target-os, abi, binary-format, architecture and address-model."
-  ./bjam toolset=gcc binary-format=pe target-os=windows threadapi=win32 threading=multi variant=release link=static --user-config=user-config.jam --without-mpi --without-python -sNO_BZIP2=1 -sNO_ZLIB=1 --layout=tagged --build-type=complete --prefix="$INSTALLPREFIX" $MAKEOPTS install
+  ./bjam toolset=gcc binary-format=pe target-os=windows threadapi=win32 threading=multi variant=release link=static runtime-link=static --user-config=user-config.jam --without-mpi --without-python -sNO_BZIP2=1 -sNO_ZLIB=1 --layout=tagged --build-type=complete --prefix="$INSTALLPREFIX" $MAKEOPTS install
 
   cd "$INSTALLPREFIX"
   export LD_PRELOAD=/usr/lib/faketime/libfaketime.so.1
   export FAKETIME=$REFERENCE_DATETIME
-  zip -r boost-win32-1.55.0-gitian-r6.zip *
-  cp boost-win32-1.55.0-gitian-r6.zip $OUTDIR
+  zip -r $OUTDIR/boost-win32-1.55.0-gitian-r6.zip *