Suggested by Jim Meyering.
http://lists.gnu.org/archive/html/bug-gnulib/2012-12/msg00017.html
* configure.ac (warn_common): Remove -Wcast-qual.
[enable_gcc_warnings=no])
if test "$enable_gcc_warnings" = yes; then
warn_common='-Wall -Wextra -Wno-sign-compare -Wcast-align
- -Wcast-qual -Wformat -Wpointer-arith -Wwrite-strings'
+ -Wformat -Wpointer-arith -Wwrite-strings'
warn_c='-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wshadow -Wstrict-prototypes'
warn_cxx='-Wnoexcept'