pgsql: Use single quotes in preference to double quotes for protecting

Use single quotes in preference to double quotes for protecting pathnames.

Per recommendation from Peter.  Neither choice is bulletproof, but this
is the existing style and it does help prevent unexpected environment
variable substitution.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/1568fa75bcf393977425ccb73752b98883a9f375

Modified Files
--------------
src/Makefile.global.in            |    4 ++--
src/Makefile.shlib                |    2 +-
src/backend/Makefile              |   20 ++++++++++----------
src/interfaces/ecpg/test/Makefile |    6 +++---
src/makefiles/Makefile.aix        |    4 ++--
src/test/isolation/Makefile       |    2 +-
6 files changed, 19 insertions(+), 19 deletions(-)