pidgin: fix build on High Sierra

On macOS High Sierra ./configure does not detect the perl_run() function
and therefore errors out. This skips the problematic test.

Compilation still works. Perl plugin support has been verified to work
with the Perl Test Plugin from
https://docs.pidgin.im/pidgin/2.x.y/perl-howto.html

Signed-off-by: Konrad Gräfe <konradgraefe@aol.com>

Closes #43067.

Signed-off-by: Chongyu Zhu <i@lembacon.com>

Formula/pidgin.rb

@@ -49,6 +49,8 @@ class Pidgin < Formula
       --without-x
     ]
 
+    ENV["ac_cv_func_perl_run"] = "yes" if MacOS.version == :high_sierra
+
     system "./configure", *args
     system "make", "install"