pgsql: Refactor installation of extension headers.

Refactor installation of extension headers.

Commit be54b3777 failed on gmake 3.80 due to a chained conditional,
which on closer examination could be removed entirely with some
refactoring elsewhere for a net simplification and more robustness
against empty expansions. Along the way, add some more comments.

Also make explicit in the documentation and comments that built
headers are not removed by 'make clean', since we don't typically want
that for headers generated by a separate ./configure step, and it's
much easier to add your own 'distclean' rule or use EXTRA_CLEAN than
to try and override a deletion rule in pgxs.mk.

Per buildfarm member prariedog and comments by Michael Paquier, though
all the actual changes are my fault.

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/094ffd68460a8ba905df7b4eae4e9c690dc6e06f

Modified Files
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doc/src/sgml/extend.sgml | 14 ++++++++--
src/makefiles/pgxs.mk    | 73 ++++++++++++++++++++++++++++--------------------
2 files changed, 55 insertions(+), 32 deletions(-)