configure: Force mingw's ld to keep the reloc section

Binutils will always strip the relocation information from executable
files even if it needs it (dynamicbase/ASLR).  We can work around this
by using the pic-executable flag combined with setting the correct entry
point since apparently ld forgets what that should be.  This problem
affects both 32 and 64-bit mingw-w64.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -4392,6 +4392,16 @@ case $target_os in
         objformat="win32"
         ranlib=:
         enable dos_paths
+        # Lets work around some stupidity in binutils.
+        # ld will strip relocations from executables even though we need them
+        # for dynamicbase (ASLR).  Using -pie does retain the reloc section
+        # however ld then forgets what the entry point should be (oops) so we
+        # have to manually (re)set it.
+        if enabled x86_32; then
+            add_ldexeflags -Wl,--pic-executable,-e,_mainCRTStartup
+        elif enabled x86_64; then
+            add_ldexeflags -Wl,--pic-executable,-e,mainCRTStartup
+        fi
         ;;
     win32|win64)
         disable symver