make install libexec fails if configured with --disable-test

[j-ogas]

$ ./configure --prefix=$HOME/deleteme --disable-test
[...]
make  install-exec-hook
make[3]: Entering directory `/home/jogas/git/charliecloud'
### re-writing embedded paths ###
for i in /home/jogas/deleteme/bin/ch-convert \
         /home/jogas/deleteme/bin/ch-fromhost \
         /home/jogas/deleteme/bin/ch-image \
         /home/jogas/deleteme/bin/ch-run-oci \
         /home/jogas/deleteme/bin/ch-test \
         /home/jogas/deleteme/lib/charliecloud/base.sh \
         /home/jogas/deleteme/libexec/charliecloud/doctest; \
do \
  sed -Ei -e 's|^(ch_lib ?= ?).+/lib"?$|\1"/home/jogas/deleteme/lib/charliecloud"|' \
          -e 's|^(CHTEST_DIR=).+$|\1/home/jogas/deleteme/libexec/charliecloud|' \
          -e 's|^(CHTEST_EXAMPLES_DIR=).+$|\1/home/jogas/deleteme/share/doc/charliecloud/examples|' \
              $i; \
done
sed: can't read /home/jogas/deleteme/libexec/charliecloud/doctest: No such file or directory
make[3]: *** [install-exec-hook] Error 2
make[3]: Leaving directory `/home/jogas/git/charliecloud'
make[2]: *** [install-exec-am] Error 2
make[2]: Leaving directory `/home/jogas/git/charliecloud'
make[1]: *** [install-am] Error 2
make[1]: Leaving directory `/home/jogas/git/charliecloud'
make: *** [install-recursive] Error 1