pgsql: Doc: improve directions for building on macOS.

Doc: improve directions for building on macOS.

In light of recent discussions, we should instruct people to
install Apple's command line tools; installing Xcode is secondary.

Also, fix sample command for finding out the default sysroot,
as we now know that the command originally recommended can give
a result that doesn't match your OS version.

Also document the workaround to use if you really don't want
configure to select a sysroot at all.

Discussion: /messages/by-id/20210119111625.20435-1-james.hilliard1@gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4dd2c3b1c3e940b09c3b26d30cd37c66c4deda4d

Modified Files
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doc/src/sgml/installation.sgml | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)