Installing from source requires asciidoc (for a2x), probably DocBook DTD files,
GNU Make and GCC or a compatible compiler. "make" is usually GNU Make in destkop
Linux distributions, but you may have to call it as "gmake" in other systems.
Quick guide:

make
make test
make install # Usually as root.

When running make, you can set values for several variables to modify
compilation, installation and meta-information paths. If you do, you need to
include them in *every* call to make.

Variables:

* CC to set the C compiler.
* PREFIX, which is /usr/local by default.
* MANDIR, which is $(PREFIX)/share/man by default.
* INCLUDEDIR, which is $(PREFIX)/include by default.
* LIBDIR, which is $(PREFIX)/lib or $(PREFIX/lib64 by default.

Only needed in the installation step:

* DESTDIR for a base installation path, empty by default.

You could also set CFLAGS but, by default, the ones provided by Solar Designer
at crypt_blowfish/Makefile are used. Personally, I recommend to keep them as
they are.