Check for the existence of msbuild.exe as well.

install.sh

@@ -165,12 +165,15 @@ fi
 if $omnisharp_completer; then
   buildcommand="msbuild"
   if ! command_exists msbuild; then
+    buildcommand="msbuild.exe"
+    if ! command_exists msbuild.exe; then
       buildcommand="xbuild"
       if ! command_exists xbuild; then
         echo "msbuild or xbuild is required to build Omnisharp"
         exit 1
       fi
     fi
+  fi
 
   ycm_dir=`pwd`
   build_dir=$ycm_dir"/python/ycm/completers/cs/OmniSharpServer"