BUG #15243: make check shows errors,i.e. fails to find initdb and others

The following bug has been logged on the website:

Bug reference:      15243
Logged by:          Mario Emmenlauer
Email address:      mario@emmenlauer.de
PostgreSQL version: 10.4
Operating system:   Ubuntu Linux
Description:

I tried my first build of PostgreSQL 10.4 on Ubuntu Linux 16.04 today. The
build worked out of the box. However `make check` fails. I am under the
impression that it tries a "strange" path to search for build-time binaries.
But I'm not experienced enough to figure out why. Here are my
observations:

Make check aborts immediately with message:

rm -rf ./testtablespace
mkdir ./testtablespace
PATH="/tmp/Debug/Shared/postgresql-10.4/tmp_install/thirdparty/Debug/Shared/bin:$PATH"
LD_LIBRARY_PATH="/tmp/Debug/Shared/postgresql-10.4/tmp_install/thirdparty/Debug/Shared/lib:$LD_LIBRARY_PATH"
../../../src/test/regress/pg_regress --temp-instance=./tmp_check
--inputdir=. --bindir=     --dlpath=.  --schedule=./parallel_schedule  
============== creating temporary instance            ==============
============== initializing database system           ==============
pg_regress: initdb failed
Examine /tmp/Debug/Shared/postgresql-10.4/src/test/regress/log/initdb.log
for the reason.
Command was: "initdb" -D
"/tmp/Debug/Shared/postgresql-10.4/src/test/regress/./tmp_check/data"
--no-clean --no-sync >
"/tmp/Debug/Shared/postgresql-10.4/src/test/regress/log/initdb.log" 2>&1
GNUmakefile:130: recipe for target 'check' failed

In /tmp/Debug/Shared/postgresql-10.4/src/test/regress/log/initdb.log there
is just a short message saying that initdb was not found. What makes me
curious is the PATH setting above that ends in
`tmp_install/thirdparty/Debug/Shared/bin`. This whole suffix does not exist
on my machine, and actually the suffix `/thirdparty/Debug/Shared/bin` is my
target installation prefix.
Does anyone else experience issues with make check, or does this ring a bell
with anyone?

Thanks for the great work, and all the best, Mario Emmenlauer