Fix name clash when building in mingw environment.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@4484 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -806,7 +806,7 @@ if test "$mingw32" = "yes" ; then
     ffserver="no"
     network="no"
     LIBPREF=""
-    LIBSUF=".lib"
+    LIBSUF=".a"
     SLIBPREF=""
     SLIBSUF=".dll"
     EXESUF=".exe"