winbuild: fix boringssl build

Compile with `WIN32_LEAN_AND_MEAN` which prevents `windows.h` from
including too much clutter including `wincrypt.h` which in turn contains
some preprocessor macros that clash with boringssl symbols.

Detect boringssl by checking the existance of `is_boringssl.h` and set
the corresponding `HAVE_BORINGSSL` for compilation which is used in
`ldap.c` to undefine the evil macros.

Closes #1610