Add gsl package

packages/gsl.rb

+require 'package'
+
+class Gsl < Package
+  description 'The GNU Scientific Library (GSL) is a numerical library for C and C++ programmers.'
+  homepage 'https://www.gnu.org/software/gsl/'
+  version '2.4'
+  source_url 'https://ftpmirror.gnu.org/gsl/gsl-2.4.tar.gz'
+  source_sha256 '4d46d07b946e7b31c19bbf33dda6204d7bedc2f5462a1bae1d4013426cd1ce9b'
+
+  binary_url ({
+  })
+  binary_sha256 ({
+  })
+
+  def self.build
+    system './configure',
+           "--prefix=#{CREW_PREFIX}",
+           "--libdir=#{CREW_LIB_PREFIX}",
+           '--disable-maintainer-mode'
+    system "make"
+  end
+
+  def self.install
+    system "make", "DESTDIR=#{CREW_DEST_DIR}", "install"
+  end
+end