Fix build on Mingw/Cygwin (fixes #144)

[rasa]

No description provided.

[rasa]

Executables are at:
https://github.com/rasa/git-crypt/releases
and can be installed via scoop:
scoop install git-crypt

[ezk84]

I want to add here that in order to make this work I had to link against the dll version of the crypto library:

make would run into a stack of errors of the form:

C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/9.1.0/../../../../lib\libcrypto.a(b_addr.o):(.text+0xab): undefined reference to `__imp_getnameinfo'

What I did then was run

g++ -Wall -pedantic -Wno-long-long -O2 -std=c++11 -static -o git-crypt git-crypt.o commands.o crypto.o gpg.o key.o util.o parse_options.o coprocess.o fhstream.o crypto-openssl-10.o crypto-openssl-11.o -lgdi32 /mingw64/lib/libcrypto.dll.a

in order to link directly to the dll and not /mingw64/lib/libcrypto.a. Seems something's off with the way libcrypto is compiled (according to this stack overflow)

Hoping this helps someone trying to get this compiled on Windows.