build: don't define DLL_EXPORT for windows

This fixes libbitcoinkernel dll linking.

Author: theuni

configure.ac

@@ -76,6 +76,12 @@ dnl we have those under control, re-enable that functionality.
 case $host in
   *mingw*)
      lt_cv_deplibs_check_method="pass_all"
+
+     dnl Remove unwanted -DDLL_EXPORT from these variables.
+     dnl We do not use this macro, but system headers may export unwanted symbols
+     dnl if it's set.
+     lt_cv_prog_compiler_pic="-DPIC"
+     lt_cv_prog_compiler_pic_CXX="-DPIC"
   ;;
 esac