Remove gcc import libraries from Win32 builds.

They weren't even installed, and [1] suggests linking directly to the dll.
[1] http://sourceware.org/binutils/docs-2.18/ld/WIN32.html

Originally committed as revision 10740 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1184,7 +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:$(SLIBSUF)=.lib)"'
-    SHFLAGS='-shared -Wl,--output-def,$(@:$(SLIBSUF)=.def),--out-implib,lib$(SLIBNAME:$(SLIBSUF)=.dll.a) -Wl,--enable-runtime-pseudo-reloc -Wl,--enable-auto-image-base'
+    SHFLAGS='-shared -Wl,--output-def,$(@:$(SLIBSUF)=.def) -Wl,--enable-runtime-pseudo-reloc -Wl,--enable-auto-image-base'
     ;;
   cygwin*)
     targetos=cygwin
@@ -1200,7 +1200,7 @@ case $targetos in
     SLIBSUF=".dll"
     SLIBNAME_WITH_VERSION='$(SLIBPREF)$(NAME)-$(LIBVERSION)$(SLIBSUF)'
     SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(NAME)-$(LIBMAJOR)$(SLIBSUF)'
-    SHFLAGS='-shared -Wl,--out-implib=lib$(NAME).dll.a -Wl,--enable-auto-image-base'
+    SHFLAGS='-shared -Wl,--enable-auto-image-base'
     ;;
   linux)
     LDLATEFLAGS="-Wl,--as-needed $LDLATEFLAGS"