lha: fix Xcode 12 build issues

There are actually two separate problems that got triggered by
Xcode 12's decision to make -Werror,-Wimplicit-function-declaration
the default:

* The configure script has problems.  Some of these can be fixed
  with an autoreconf run, but the non-HEAD version of the code
  needs other adjustments to work with modern autoconf.  However
  there are a couple other problems which I opened an upstream PR
  for already: jca02266/lha#18

* There is also a build issue.  macports already has a fix for that:
  macports/macports-ports#9197

However the simplest thing to do for the moment is to just disable
the warning via $CFLAGS which allows the code to build as-is

Closes #66361.

Signed-off-by: FX Coudert <fxcoudert@gmail.com>
Signed-off-by: BrewTestBot <1589480+BrewTestBot@users.noreply.github.com>

Formula/lha.rb

@@ -37,6 +37,9 @@ class Lha < Formula
   conflicts_with "lhasa", because: "both install a `lha` binary"
 
   def install
+    # Work around configure/build issues with Xcode 12
+    ENV.append "CFLAGS", "-Wno-implicit-function-declaration"
+
     system "autoreconf", "-is" if build.head?
     system "./configure", "--disable-debug",
                           "--disable-dependency-tracking",