autotools: fix subdir m4 check, never let it return -I

If BDB_CPPFLAGS returns only "-I", the next argument sent to the preprocessor
is treated as a path. There are 2 fixes here:

1. Check in CPPFLAGS, as a user might have manually passed a path to check.
2. Ensure the value is not empty before setting BDB_CPPFLAGS to "-I value"
autotools: fix the Makefile.include to be safely included anywhere.

This way we can reuse rules rather than duplicating them.
win32: add version info to bitcoind.exe

TODO: Add icon info
win32: only check for windres for win32, and fail if missing
Merge pull request #3015 from theuni/win32-version-info

Win32 version info
Import libsecp256k1
Support for libsecp256k1
gitian support for secp256k1 linux builds