configure: use dlltools instead of lib.exe

This way building ffmpeg on mingw won't require windows specific tools

configure

@@ -2415,7 +2415,7 @@ case $target_os in
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(FULLNAME)-$(LIBMAJOR)$(SLIBSUF)'
-        SLIB_EXTRA_CMD='-lib.exe /machine:$(LIBTARGET) /def:$$(@:$(SLIBSUF)=.def) /out:$(SUBDIR)$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib)'
+        SLIB_EXTRA_CMD=-'$(DLLTOOL) -m $(LIBTARGET) -d $$(@:$(SLIBSUF)=.def) -l $(SUBDIR)$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib)'
         SLIB_INSTALL_EXTRA_CMD='-install -m 644 $(SUBDIR)$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib) "$(SHLIBDIR)/$(SLIBNAME:$(SLIBSUF)=.lib)"; \
             install -m 644 $(SUBDIR)$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib) "$(SHLIBDIR)/$(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.lib)"; \
             install -d "$(LIBDIR)"; \
@@ -2423,6 +2423,7 @@ case $target_os in
         SLIB_UNINSTALL_EXTRA_CMD='rm -f "$(SHLIBDIR)/$(SLIBNAME:$(SLIBSUF)=.lib)"'
         SHFLAGS='-shared -Wl,--output-def,$$(@:$(SLIBSUF)=.def) -Wl,--out-implib,$(SUBDIR)lib$(SLIBNAME:$(SLIBSUF)=.dll.a) -Wl,--enable-runtime-pseudo-reloc -Wl,--enable-auto-image-base'
         objformat="win32"
+        dlltool="${cross_prefix}dlltool"
         enable dos_paths
         check_cflags -fno-common
         check_cpp_condition _mingw.h "defined (__MINGW64_VERSION_MAJOR) || (__MINGW32_MAJOR_VERSION > 3) \
@@ -3232,6 +3233,7 @@ CPPFLAGS=$CPPFLAGS
 CFLAGS=$CFLAGS
 ASFLAGS=$ASFLAGS
 CC_O=$CC_O
+DLLTOOL=$dlltool
 LDFLAGS=$LDFLAGS
 FFSERVERLDFLAGS=$FFSERVERLDFLAGS
 SHFLAGS=$SHFLAGS