macOS distributions don't provide gprbuild, others do.

  * index/gp/gprbuild/gprbuild-external.toml: the "system" external isn't
    available on macOS, so specify it explicitly for the other operating
    systems.

index/gp/gprbuild/gprbuild-external.toml

@@ -9,3 +9,10 @@ kind = "version-output"
 version-regexp = "^GPRBUILD ([\\d\\.-]+).*|^GPRBUILD Community ([\\d\\.-]+).*"
 version-command = ["gprbuild", "--version"]
 
+# Neither macOS distribution (Homebrew, MacPorts) provides gprbuild.
+[[external]]
+kind = "system"
+[external.origin.'case(os)']
+"freebsd" = ["gprbuild"]
+"linux"   = ["gprbuild"]
+"windows" = ["gprbuild"]