appveyor: Fix LDFLAGS for Windows build

appveyor: Fix LDFLAGS for Windows build

With newer OpenSSL builds, the DLL files contain parts of the version
number and the architecture in their name, e.g. for OpenSSL 1.1.1 the
DLL for libcrypto is called libcrypto-1_1-x64.dll.  So referencing that
directly could be kinda tricky.  And by using `-lcrypto` we therefore
didn't link those DLLs but the OpenSSL version installed by msys2.
Since the latter ships OpenSSL 3 since January and the VS 2019 image
was updated recently, our builds broke as we used the headers from
the 1.1.1 installation but then tried to link OpenSSL 3.

Luckily, in the lib/ directory of the OpenSSL installation, there is a
libcrypto.lib file, which is an import library (containing the symbols
and a reference to the DLL).  We can use that to link the right library
via `-lcrypto`.

With the old OpenSSL 1.0.2 build on the VS 2015 image, there is also
such a .lib file but it seems the linker is too old or otherwise incapable
of finding the DLL.  But since the DLL is just called libeay32.dll there,
we use that directly and don't reference the lib/ dir.

Also removed a superfluous AC_MSG_RESULT() if libeay32 isn't found.