Add SLIB_UNINSTALL_EXTRA_CMD for MinGW.

based on a patch by Dave Yeo, daveryeo telus net

Originally committed as revision 10768 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1184,6 +1184,7 @@ case $targetos in
     SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(NAME)-$(LIBMAJOR)$(SLIBSUF)'
     SLIB_EXTRA_CMD='-lib /machine:i386 /def:$(@:$(SLIBSUF)=.def)'
     SLIB_INSTALL_EXTRA_CMD='-install -m 644 $(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib) "$(SHLIBDIR)/$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib)"'
+    SLIB_UNINSTALL_EXTRA_CMD='rm -f "$(SHLIBDIR)/$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib)"'
     SHFLAGS='-shared -Wl,--output-def,$(@:$(SLIBSUF)=.def) -Wl,--enable-runtime-pseudo-reloc -Wl,--enable-auto-image-base'
     ;;
   cygwin*)