Getting the source
The ECWolf source code is available in both a 7z archive on the website or can be download directly from the repository. The Mercurial repository is hosted on Bitbucket.
The code in ECWolf is licensed under a variety of licenses. The zlib code uses it's own license and a few dependencies are LGPL. If not compiling for the GPL, the MAME license also applies.
All new code, be it from ZDoom or ECWolf itself, falls under a 3-point BSD.
Wolfenstein 3D License
ECWolf is dual licensed under both the original non-commercial license or the GNU GPL. The GPL but can be switched off with -DGPL=OFF in cmake. The two versions are identical except the OPL emulator is changed to MAME's from the one from Dosbox.
It is important to note that id Software has not done any packaged re-releases of the Wolf3D source under the GPL, but Doom has not been officially re-released either. The following quote from John Carmack dated March 20th, 2009 seems to indicate that use of the GPL has been allowed since Doom was re-licensed.
I released the original source for Wolfenstein 3D many years ago, originally under a not-for-commercial purposes license, then later under the GPL.
The Wolf4SDL debian packages also confirms the GPL re-release of Wolfenstein 3D.