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.