A 64-bit core can be built using w64-mingw.

configure.ac

@@ -303,17 +303,21 @@ AS_IF([test "x$enable_static_linking" = "xyes"], [
       )
     ],
     [mingw*], [
-      CORE_CPPFLAGS='-I$(top_srcdir)/static-libs/libs-built-32/windows '"${CORE_CPPFLAGS}"
+      CONFIGURE_ARGS="--host=\"$host_alias\" --target=\"$target_alias\""
+      OPENSSL_ARGS_32="--cross-compile-prefix=\"$host_alias-\" mingw enable-static-engine ${LIB_CFLAGS_32}"
+      OPENSSL_ARGS_64="--cross-compile-prefix=\"$host_alias-\" mingw64 enable-static-engine ${LIB_CFLAGS_64}"
+      CROSSCOMPILE_PREPARATION_EXPORTS="export CC=\"${host_alias}-gcc\" && export LD=\"${host_alias}-ld\" && "
+      CORE_CPPFLAGS_32='-I$(top_srcdir)/static-libs/libs-built-32/windows '"${CORE_CPPFLAGS_32}"
+      CORE_CPPFLAGS_64='-I$(top_srcdir)/static-libs/libs-built-64/windows '"${CORE_CPPFLAGS_64}"
       AC_DEFINE(TGT_SYSTEM, TGT_WIN, [Target system is Windows.])
       AC_DEFINE(__USE_MINGW_ANSI_STDIO, 1, [MinGW requires this for GNU format specifiers?])
       AS_CASE([${BITS}],
         [32], [
-          CONFIGURE_ARGS="--host=\"$host_alias\" --target=\"$target_alias\""
-          OPENSSL_ARGS_32="--cross-compile-prefix=\"$host_alias-\" mingw enable-static-engine ${LIB_CFLAGS_32}"
-          CROSSCOMPILE_PREPARATION_EXPORTS="export CC=\"${host_alias}-gcc\" && export LD=\"${host_alias}-ld\" && "
+        ],
+        [64], [
         ],
         [
-          AC_MSG_ERROR([You can only build 32-bit applications using MinGW.])
+          AC_MSG_ERROR([You can only build 32-bit or 64-bit applications using MinGW.])
         ]
       )
     ]

static-libs/Makefile.am

@@ -114,6 +114,12 @@ COPY_HEADERS_CMD_32 = \
 	$(INSTALL) -d $(abs_builddir)/libs-built-32/windows/ldns && \
 	$(INSTALL) -m 644 ldns/config.h $(abs_builddir)/libs-built-32/windows/ldns/config.h && \
 	$(INSTALL) -m 644 ldns/net.h $(abs_builddir)/libs-built-32/windows/ldns/net.h)
+
+COPY_HEADERS_CMD_64 = \
+	(cd libs-srcs-64/ldns && \
+	$(INSTALL) -d $(abs_builddir)/libs-built-64/windows/ldns && \
+	$(INSTALL) -m 644 ldns/config.h $(abs_builddir)/libs-built-64/windows/ldns/config.h && \
+	$(INSTALL) -m 644 ldns/net.h $(abs_builddir)/libs-built-64/windows/ldns/net.h)
 endif
 
 # Linker from mingw-w64 has problems passing the linktest of ldns-1.6.17