SSH2.c:1103:14: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

This is a fairly common warning with perls where perl -V:ivsize and perl -V:ptrsize output different values, but it shouldn't arise when the 2 values are the same.
(Eg, it shouldn't arise on 32-bit perls built with 32-bit integers.)

SSH2.o:SSH2.c:(.text+0x14f8d): undefined reference to `CRYPTO_set_dynlock_destroy_callback'

You said that this happens "sometimes", but hopefully it's not random.
It looks like a problem with the linking command - perhaps a missing or mis-located -leay32.
(With the Strawberry Perl distro, the symbol is defined in libeay32.a.)

What are the conditions that determine whether or not you get that error ?

whereas ActivePerl used Microsoft compilers. But this has turned out to be WRONG! Both use gcc.

Prior to perl-5.18.0, ActivePerl was built using Microsoft's MSVC 6.0 compiler - but since then they've been building with gcc.
However, even with the MSVC-built perls (since about perl-5.10.0), it has been straightforward to use gcc with ActivePerl.

Cheers,
Rob