BUG #15349: "make installcheck -C src/interfaces/libpq" fails

The following bug has been logged on the website:

Bug reference: 15349
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 11beta3
Operating system: Ubuntu 18.04
Description:

With REL_11_STABLE (also reproduced with versions 10 and 9.6) "make
installcheck -C src/interfaces/libpq" fails:
./configure && make
make installcheck -C src/interfaces/libpq # or make all -C
src/interfaces/libpq/test/

gcc -L../../../../src/port -L../../../../src/common -L../../../../src/common
-lpgcommon -L../../../../src/port -lpgport
-L../../../../src/interfaces/libpq -lpq -Wl,--as-needed
-Wl,-rpath,'/usr/local/pgsql/lib',--enable-new-dtags uri-regress.o -o
uri-regress
uri-regress.o: In function `main':
uri-regress.c:(.text.startup+0x60): undefined reference to
`PQconninfoParse'
uri-regress.c:(.text.startup+0x71): undefined reference to
`PQconndefaults'
collect2: error: ld returned 1 exit status
<builtin>: recipe for target 'uri-regress' failed
make: *** [uri-regress] Error 1

If I restore previous version of libpq/test/Makefile:
git show dddfc4cb~1:src/interfaces/libpq/test/Makefile
>src/interfaces/libpq/test/Makefile
then "make all -C src/interfaces/libpq/test/" succeeds:
gcc -L../../../../src/port -L../../../../src/common -Wl,--as-needed
-Wl,-rpath,'/usr/local/pgsql/lib',--enable-new-dtags uri-regress.o
-L../../../../src/common -lpgcommon -L../../../../src/port -lpgport
-L../../../../src/interfaces/libpq -lpq -o uri-regress
make: Leaving directory '/space/tmp/postgres.chk/src/interfaces/libpq/test'