makedef: Pass EXTERN_PREFIX from configure to makedef

This avoids having to use either "dumpbin -headers" to find out
the current architecture, or pass $ARCH from configure to deduce it.

When configuring with --disable-asm, ARCH is equal to "c", which doesn't
give any indication of what symbol prefix is to be used.

Signed-off-by: Martin Storsjö <martin@martin.st>

compat/windows/makedef

@@ -58,33 +58,7 @@ fi
 IFS='
 '
 
-# Determine if we're building for x86 or x86_64 and
-# set the symbol prefix accordingly.
-prefix=""
-if [ -n "$NM" ]; then
-    case $ARCH in
-    *86)
-        prefix="_"
-        ;;
-    *)
-        ;;
-    esac
-else
-    arch=$(dumpbin -headers ${libname} |
-           tr '\t' ' ' |
-           grep '^ \+.\+machine \+(.\+)' |
-           head -1 |
-           sed -e 's/^ \{1,\}.\{1,\} \{1,\}machine \{1,\}(\(.\{3,5\}\)).*/\1/')
-
-    if [ "${arch}" = "x86" ]; then
-        prefix="_"
-    else
-        if [ "${arch}" != "ARM" ] && [ "${arch}" != "x64" ] && [ "${arch}" != "ARM64" ]; then
-            echo "Unknown machine type." >&2
-            exit 1
-        fi
-    fi
-fi
+prefix="$EXTERN_PREFIX"
 
 started=0
 regex="none"