postgresql@9.6: fix Mojave build

Formula/postgresql@9.6.rb

@@ -82,11 +82,42 @@ class PostgresqlAT96 < Formula
     args << "--enable-dtrace" if build.with? "dtrace"
     args << "--with-uuid=e2fs"
 
+    # As of Xcode/CLT 10.x the Perl headers were moved from /System
+    # to inside the SDK, so we need to use `-iwithsysroot` instead
+    # of `-I` to point to the correct location.
+    # https://www.postgresql.org/message-id/153558865647.1483.573481613491501077%40wrigleys.postgresql.org
+    if DevelopmentTools.clang_build_version >= 1000
+      inreplace "configure",
+                "-I$perl_archlibexp/CORE",
+                "-iwithsysroot $perl_archlibexp/CORE"
+      inreplace "contrib/hstore_plperl/Makefile",
+                "-I$(perl_archlibexp)/CORE",
+                "-iwithsysroot $(perl_archlibexp)/CORE"
+      inreplace "src/pl/plperl/GNUmakefile",
+                "-I$(perl_archlibexp)/CORE",
+                "-iwithsysroot $(perl_archlibexp)/CORE"
+    end
+
     system "./configure", *args
     system "make"
-    system "make", "install-world", "datadir=#{pkgshare}",
-                                    "libdir=#{lib}",
-                                    "pkglibdir=#{lib}"
+
+    dirs = %W[datadir=#{pkgshare} libdir=#{lib} pkglibdir=#{lib}]
+
+    # Temporarily disable building/installing the documentation.
+    # Postgresql seems to "know" the build system has been altered and
+    # tries to regenerate the documentation when using `install-world`.
+    # This results in the build failing:
+    #  `ERROR: `osx' is missing on your system.`
+    # Attempting to fix that by adding a dependency on `open-sp` doesn't
+    # work and the build errors out on generating the documentation, so
+    # for now let's simply omit it so we can package Postgresql for Mojave.
+    if DevelopmentTools.clang_build_version >= 1000
+      system "make", "all"
+      system "make", "-C", "contrib", "install", "all", *dirs
+      system "make", "install", "all", *dirs
+    else
+      system "make", "install-world", *dirs
+    end
   end
 
   def post_install