Emit dummy statements for probes.d probes when disabled

When building without --enable-dtrace, emit dummy

    do {} while (0)

statements for the stubbed-out TRACE_POSTGRESQL_foo() macros
instead of empty macros that totally elide the original probe
statement.

This fixes the

    warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

introduced by b94409a02f.

Author: Craig Ringer <craig.ringer@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/20210504221531.cfvpmmdfsou6eitb%40alap3.anarazel.de

src/backend/utils/Gen_dummy_probes.pl

@@ -135,6 +135,12 @@ sub Run()
 			$CondReg ||= $s;
 		}
 
+		# s/$/ do {} while (0)/
+		{
+			$s = s /$/ do {} while (0)/s;
+			$CondReg ||= $s;
+		}
+
 		# P
 		{
 			if (/^(.*)/) { print $1, "\n"; }

src/backend/utils/Gen_dummy_probes.sed

@@ -19,5 +19,6 @@ s/([^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\})/(INT1, INT2,
 s/([^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\})/(INT1, INT2, INT3, INT4, INT5, INT6)/
 s/([^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\})/(INT1, INT2, INT3, INT4, INT5, INT6, INT7)/
 s/([^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\}, [^,)]\{1,\})/(INT1, INT2, INT3, INT4, INT5, INT6, INT7, INT8)/
+s/$/ do {} while (0)/
 P
 s/(.*$/_ENABLED() (0)/