FFmpeg configure: Update SLIBPREF on windows

- fixes mingw FFmpeg linking and aligns with the values used in the
master configure

Refs #293

mythtv/external/FFmpeg/configure

@@ -5403,7 +5403,7 @@ case $target_os in
         enabled shared && ! enabled small && test_cmd $windres --version && enable gnu_windres
         enabled x86_32 && check_ldflags -Wl,--large-address-aware
         shlibdir_default="$bindir_default"
-        SLIBPREF=""
+        SLIBPREF="libmyth"
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(FULLNAME)-$(LIBMAJOR)$(SLIBSUF)'
@@ -5452,7 +5452,7 @@ case $target_os in
         fi
         enabled x86_32 && check_ldflags -LARGEADDRESSAWARE
         shlibdir_default="$bindir_default"
-        SLIBPREF=""
+        SLIBPREF="libmyth"
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(FULLNAME)-$(LIBMAJOR)$(SLIBSUF)'
@@ -5503,7 +5503,7 @@ case $target_os in
         add_ldflags -Zomf -Zbin-files -Zargs-wild -Zhigh-mem -Zmap
         SHFLAGS='$(SUBDIR)$(NAME).def -Zdll -Zomf'
         LIBSUF="_s.a"
-        SLIBPREF=""
+        SLIBPREF="myth"
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(FULLNAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(shell echo $(FULLNAME) | cut -c1-6)$(LIBMAJOR)$(SLIBSUF)'