pgsql: Allow extensions to install built as well as unbuilt headers.

Allow extensions to install built as well as unbuilt headers.

Commit df163230b overlooked the case that an out-of-tree extension
might need to build its header files (e.g. via ./configure). If it is
also doing a VPATH build, the HEADERS_* rules in the original commit
would then fail to find the files, since they would be looking only
under $(srcdir) and not in the build directory.

Fix by adding HEADERS_built and HEADERS_built_$(MODULE) which behave
like DATA_built in that they look in the build dir rather than the
source dir (and also make the files dependencies of the "all" target).

No Windows support appears to be needed for this, since it is only
relevant to out-of-tree builds (no support exists in Mkvcbuild.pm to
build extension header files in any case).

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/235526a20a2efb199c4138c5d793260ce37d7231

Modified Files
--------------
doc/src/sgml/extend.sgml | 16 +++++++++++++---
src/makefiles/pgxs.mk | 49 +++++++++++++++++++++++++++++++++++++++---------
2 files changed, 53 insertions(+), 12 deletions(-)