pgsql: meson: Rename cpp variable to cxx

meson: Rename cpp variable to cxx

Since CPP is also used to mean C PreProcessor in our meson.build
files, it's confusing to use it to also mean C PlusPlus.  This uses
the non-ambiguous cxx wherever possible.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/D98JHQF7H2A8.VSE3I4CJBTAB%40partin.io

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/1f08d687c39607d8dfa4bfd0fcb1a6ad47a1ef56

Modified Files
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meson.build             | 18 +++++++++---------
src/backend/meson.build |  4 ++--
2 files changed, 11 insertions(+), 11 deletions(-)