meson: Move C99 test earlier

meson: Move C99 test earlier

Move the test for compiler options for C99 earlier in meson.build,
before we make use of the compiler for other tests.  That way, if any
command-line options are needed, subsequent tests will also use them.
This is at the moment a theoretical problem, but it seems better to
get this correct.  It also matches the order in the Autoconf-based
build more closely.

Discussion: https://www.postgresql.org/message-id/flat/01a69441-af54-4822-891b-ca28e05b215a@eisentraut.org