Fix MinGW 64bit compiling

1. Fix bootstrap constant names so static option is used
2. Fix ax_compiler_version to look for 64bit MinGW instead of 32bit
3. Remove PIE from ax_harden_compiler_flags for MinGW

MinGW doesn't support -fPIE at all and -pie relocates main() so MinGW's
own dummy main() becomes the entry point for the executable (and
therefore the exec does nothing).

Fixes #6