Currently the core of radare can be compiled in many systems, and architectures, but the main development is done on GNU/Linux and GCC. But it is known to compile with TCC and SunStudio.
People usually wants to use radare as a debugger for reverse engineering, and this is a bit more restrictive portability issue, so if the debugger is not ported to your favorite platform, please, notify it to me or just disable the debugger layer with --without-debugger in the ./configure stage.
Nowadays the debugger layer can be used on Windows, GNU/Linux (intel32, intel64, mips, arm), FreeBSD, NetBSD, OpenBSD (intel32, intel64) and there are plans for Solaris and OSX. And there are some IO plugins to use gdb, gdbremote or wine as backends.
The current build system is 'waf':
$ ./waf distclean $ ./waf configure $ ./waf $ sudo ./waf install ...
The old build system based on ACR/GMAKE stills maintained and usable, but don't relay on it because it is aimed to be removed to only use waf.
$ ./configure --prefix=/usr $ gmake $ sudo gmake install