The following failure will occur when building with Clang:

libtool: compile: clang -DHAVE_CONFIG_H -I. -I.. -I. -I../include -Iinclude -I../src -DFFI_BUILDING -g -O2 -pipe -march=amdfam10 -mtune=amdfam10 -Wall -fexceptions -c ../src/x86/ffi64.c -fPIC -DPIC -o src/x86/.libs/ffi64.o /bin/bash ./libtool --tag=CC --mode=compile clang -DHAVE_CONFIG_H -I. -I.. -I. -I../include -Iinclude -I../src -DFFI_BUILDING -g -O2 -pipe -march=amdfam10 -mtune=amdfam10 -Wall -fexceptions -c -o src/x86/ffi.lo ../src/x86/ffi.c ../src/x86/unix64.S:357:17: error: invalid variant 'rel' .long .LUW0@rel ^ ../src/x86/unix64.S:399:17: error: invalid variant 'rel' .long .LUW5@rel ^

I discovered this issue on Gentoo. The Mozilla developers wrote a patch for it. Further information can be found in the downstream bug trackers:

https://bugs.gentoo.org/show_bug.cgi?id=417179
https://bugzilla.mozilla.org/show_bug.cgi?id=631928