Currently in some places of `configure.ac' `${srcdir}' is used without surrounding double quotes what results in `configure' failure. The attached patch makes usage of `${srcdir}' more consistent and fixes it. Yes, I know that when one enter `configure' phase with `${srcdir}' which contains whitespace, one would see

checking whether build environment is sane... configure: error: unsafe srcdir value: ...

anyway. But that's another story. Maybe on some bright day, it would be finally safe according to Autotools. Call this pedantic, but I think it is better to receive this official prohibition error, rather than confusing

configure: line 3557: ...: No such file or directory

for `${srcdir}' with whitespace.