You beat me to it!
I was able to compile using the -mno-sse2 flag (just to be sure) after making changes similar to yours in the "sigil-0.7.2/src/src/Xerces/xercesc/util/Xerces_autoconf_config.hpp" file.
(I basically just downloaded the xerces source and ran configure using the --disable-sse2 switch to see what it produced, and then compared it to the source in Sigil)
No errors and it still runs on my machine, but I have no hardware that DOESN'T support SSE2 to truly test it on.
I created a patch to change those two files if it makes it any easier. Just drop it in the uppermost level of Sigil's source tree (sigil-0.7.2/src if you're using the official instructions), cd to that same location and issue:
patch -p1 < sigil-0.7.2-no-sse2.patch
before running cmake.
(I attached it as a txt file so it would let me upload it ... you can rename it to sigil-0.7.2-no-sse2.patch or modify the command accordingly.)