Add option for 8192 bit GPG keys.

Although it is technicaly possible to work with 8192 bit GPG keys the
creators of Gnupgp ship the sources with a hard coded limit of 4096
bit. If you want a larger GPG key than 4096 bit you're supposed to
change the given limit yourself by changing one line of code in the file
g10/keygen.c. That's it. After that you can create 8192 bit GPG keys.

Closes Homebrew/homebrew#4201.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/gnupg.rb

@@ -17,7 +17,10 @@ class Gnupg <Formula
   end
 
   def options
-    [["--idea", "Build with (patented) IDEA cipher"]]
+    [
+      ["--idea", "Build with (patented) IDEA cipher"],
+      ["--8192", "Build with support for private keys up to 8192 bits"],
+    ]
   end
 
   def install
@@ -28,6 +31,8 @@ class Gnupg <Formula
       system 'gunzip', 'cipher/idea.c.gz'
     end
 
+    inreplace 'g10/keygen.c', 'max=4096', 'max=8192' if ARGV.include? '--8192'
+
     system "/usr/bin/autoconf"
 
     system "./configure", "--disable-dependency-tracking",