Libeasyrsa is a project mainly written in D and C, it's free.
Wrapper over polarssl/rsa.h (C and D headers).
easyrsa is just C/D wrapper over polarssl which allows manipulation with keys and signing/verifing messages.
First of all, rename Makefile.xxx (depends on your platform) to just "Makefile".
Then run: make download # downloads polarssl from svn (you have to have svn installed in PATH) make # create out/libeasyrsa.a (or out/libeasyrsa.lib)
Makefile expects installed msys (http://www.mingw.org/wiki/MSYS), mingw-gcc and latest dmd (2.054).
On windows, both libraries (polarssl & easyrsa) are converted into OMF object format. That is because DMD can't handle COFF, so you can't use GCC (which can't handle OMF :S) for linking into C program.
If you want to use easyrsa with gcc on windows (COFF), run these commands:
gcc easyrsa.c -c
ar -rcs libeasyrsa.a *.o
cp polarssl/library/*.a .