Fix up mingw32 building

Author: agoode

mingw32-cheatsheet.txt

@@ -2,4 +2,4 @@
 * Use classpath's jni.h instead of OpenJDK's, because it has all the
   win32 conditionals
 
-i686-pc-mingw32-gcc -Wl,--kill-at -fPIC -fno-strict-aliasing -shared -g -O2 -Wall -o openslidejava.dll openslide_wrap.c -I/tmp/zzzzz/usr/i686-pc-mingw32/sys-root/mingw/include/openslide -I/usr/lib/gcc/x86_64-redhat-linux/4.4.0/include -lopenslide -L/tmp/zzzzz/usr/i686-pc-mingw32/sys-root/mingw/lib
+i686-pc-mingw32-gcc -Wl,--kill-at -shared -g -O2 -Wall -Wno-pointer-to-int-cast -Wno-int-to-pointer-cast -o openslidejava.dll openslide-jni.c -I/tmp/zzzzz/usr/i686-pc-mingw32/sys-root/mingw/include/openslide -I/usr/lib/gcc/x86_64-redhat-linux/4.4.0/include -lopenslide -L/tmp/zzzzz/usr/i686-pc-mingw32/sys-root/mingw/lib

openslide-jni.c

@@ -199,7 +199,7 @@ static JNINativeMethod methods[] = {
   { "openslide_get_error", "(J)Ljava/lang/String;", (void *) osj_get_error },
 };
 
-jint JNI_OnLoad(JavaVM *vm, void *reserved) {
+JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved) {
   JNIEnv* env;
   if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK) {
     return -1;