In configure.bat the TARGET_OS define is written into
build/Makefiles/config.mak. The name is both misleading
(there is no TARGET_OS named x86) and the define is not
used for Microsoft compilers throughout the codebase in
the 5.0 CVS branch. This patch removes the writing of
the TARGET_OS define into the makefile. The patch is
for the 5.0 branch.