meson: Mark PROVE as not required

In the meson build the prove binary is currently not even used. It will soon
be, for PGXS compatibility, but even then we should build without it around.

Author: Justin Pryzby <pryzbyj@telsasoft.com>
Discussion: https://postgr.es/m/20221021034040.GT16921@telsasoft.com
Discussion: https://postgr.es/m/20221104235412.GE16921@telsasoft.com

meson.build

@@ -323,7 +323,7 @@ python = find_program(get_option('PYTHON'), required: true, native: true)
 flex = find_program(get_option('FLEX'), native: true, version: '>= 2.5.35')
 bison = find_program(get_option('BISON'), native: true, version: '>= 2.3')
 sed = find_program(get_option('SED'), 'sed', native: true)
-prove = find_program(get_option('PROVE'), native: true)
+prove = find_program(get_option('PROVE'), native: true, required: false)
 tar = find_program(get_option('TAR'), native: true)
 gzip = find_program(get_option('GZIP'), native: true)
 program_lz4 = find_program(get_option('LZ4'), native: true, required: false)