configure: Use dllexport/dllimport for data symbols across DLLs with mingw

This avoids having to use pseudo relocations.

The version script used for exporting functions is skipped as soon
as the set of object files contains symbols marked with dllexport,
therefore we need to use makedef to produce the full list of symbols
to be exported.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -4026,6 +4026,10 @@ case $target_os in
         if enabled x86_64; then
             LIBTARGET="i386:x86-64"
         fi
+        if enabled shared; then
+            # Cannot build both shared and static libs when using dllexport.
+            disable static
+        fi
         check_ldflags -Wl,--nxcompat
         check_ldflags -Wl,--dynamicbase
         shlibdir_default="$bindir_default"
@@ -4033,12 +4037,13 @@ case $target_os in
         SLIBSUF=".dll"
         SLIBNAME_WITH_VERSION='$(SLIBPREF)$(NAME)-$(LIBVERSION)$(SLIBSUF)'
         SLIBNAME_WITH_MAJOR='$(SLIBPREF)$(NAME)-$(LIBMAJOR)$(SLIBSUF)'
-        SLIB_EXTRA_CMD=-'sed -e "s/ @[^ ]*//" $$(@:$(SLIBSUF)=.orig.def) > $$(@:$(SLIBSUF)=.def); $(DLLTOOL) -m $(LIBTARGET) -d $$(@:$(SLIBSUF)=.def) -l $(SUBDIR)$(SLIBNAME:$(SLIBSUF)=.lib) -D $(SLIBNAME_WITH_MAJOR)'
+        SLIB_EXTRA_CMD=-'$(DLLTOOL) -m $(LIBTARGET) -d $$(@:$(SLIBSUF)=.def) -l $(SUBDIR)$(SLIBNAME:$(SLIBSUF)=.lib) -D $(SLIBNAME_WITH_MAJOR)'
         SLIB_INSTALL_NAME='$(SLIBNAME_WITH_MAJOR)'
         SLIB_INSTALL_LINKS=
         SLIB_INSTALL_EXTRA_SHLIB='$(SLIBNAME:$(SLIBSUF)=.lib)'
         SLIB_INSTALL_EXTRA_LIB='lib$(SLIBNAME:$(SLIBSUF)=.dll.a) $(SLIBNAME_WITH_MAJOR:$(SLIBSUF)=.def)'
-        SHFLAGS='-shared -Wl,--output-def,$$(@:$(SLIBSUF)=.orig.def) -Wl,--out-implib,$(SUBDIR)lib$(SLIBNAME:$(SLIBSUF)=.dll.a) -Wl,--enable-runtime-pseudo-reloc -Wl,--enable-auto-image-base'
+        SLIB_CREATE_DEF_CMD='ARCH="$(ARCH)" AR="$(AR_CMD)" NM="$(NM_CMD)" $(SRC_PATH)/compat/windows/makedef $(SUBDIR)lib$(NAME).ver $(OBJS) > $$(@:$(SLIBSUF)=.def)'
+        SHFLAGS='-shared -Wl,--out-implib,$(SUBDIR)lib$(SLIBNAME:$(SLIBSUF)=.dll.a) -Wl,--enable-auto-image-base $$(@:$(SLIBSUF)=.def)'
         enabled x86_64 && objformat="win64" || objformat="win32"
         dlltool="${cross_prefix}dlltool"
         ranlib=:
@@ -5432,6 +5437,8 @@ DEPX86ASMFLAGS=\$(X86ASMFLAGS)
 AR=$ar
 ARFLAGS=$arflags
 AR_O=$ar_o
+AR_CMD=$ar
+NM_CMD=$nm
 RANLIB=$ranlib
 STRIP=$strip
 LN_S=$ln_s

libavcodec/internal.h

@@ -285,7 +285,7 @@ int ff_decode_frame_props(AVCodecContext *avctx, AVFrame *frame);
  */
 AVCPBProperties *ff_add_cpb_side_data(AVCodecContext *avctx);
 
-#if defined(_MSC_VER) && CONFIG_SHARED
+#if defined(_WIN32) && CONFIG_SHARED
 #ifdef BUILDING_avcodec
 #    define av_export_avcodec __declspec(dllexport)
 #else