gen_msvs_vcxproj.sh: add --enable-werror option

translates to TreatWarningAsError (/WX)
setting this via the CL environment variable is not possible due to the
/WX- default which is used on the command line

Change-Id: I0b42a9d3ca9eba6af82c25b8e434baa2fcb00156
diff --git a/configure b/configure
index d2f17b0..170bd8d 100755
--- a/configure
+++ b/configure
@@ -689,6 +689,7 @@
              10|11|12)
                  VCPROJ_SFX=vcxproj
                  gen_vcproj_cmd=${source_path}/build/make/gen_msvs_vcxproj.sh
+                 enabled werror && gen_vcproj_cmd="${gen_vcproj_cmd} --enable-werror"
                  ;;
              esac
              all_targets="${all_targets} solution"