INSTALL: clarify --cross-compile-prefix.

  --cross-compile-prefix=PREFIX

                   The PREFIX to include in front of commands for your

                   toolchain. For example to build the mingw64 target on Linux

                   you might use "--cross-compile-prefix=x86_64-w64-mingw32-".

                   If the compiler is gcc, then this will attempt to run

                   x86_64-w64-mingw32-gcc when compiling.

                   toolchain. It's likely to have to end with dash, e.g.

                   a-b-c- would invoke GNU compiler as a-b-c-gcc, etc.

                   Unfortunately cross-compiling is too case-specific to

                   put together one-size-fits-all instructions. You might

                   have to pass more flags or set up environment variables

                   to actually make it work. Android and iOS cases are

                   discussed in corresponding Configurations/10-main.cf

                   sections. But there are cases when this option alone is

                   sufficient. For example to build the mingw64 target on

                   Linux "--cross-compile-prefix=x86_64-w64-mingw32-"

                   works. Naturally provided that mingw packages are

                   installed. Today Debian and Ubuntu users have option to

                   install a number of prepackaged cross-compilers along

                   with corresponding run-time and development packages for

                   "alien" hardware. To give another example

                   "--cross-compile-prefix=mipsel-linux-gnu-" suffices

                   in such case. Needless to mention that you have to

                   invoke ./Configure, not ./config, and pass your target

                   name explicitly.

  --debug

                   Build OpenSSL with debugging symbols.