pgsql: Reduce worst-case shell command line length during "makeinstall

Reduce worst-case shell command line length during "make install".

Addition of the catalog/pg_foo_d.h headers seems to have pushed us over
the brink of the maximum command line length for some older platforms
during "make install" for our header files.  The main culprit here is
repetition of the target directory path, which could be long.
Rearrange so that we don't repeat that once per file, but only once
per subdirectory.

Per buildfarm.

Discussion: https://postgr.es/m/E1f5Dwm-0004n5-7O@gemulon.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cca563f38422f2a7c6c56f162efe2689bf1c7697

Modified Files
--------------
src/include/Makefile | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)