Add gcc 7.3 package. (#2173)

* Add gcc 7.3 package.

* Update dependencies and remove x86_64 specifics in gcc7.

* Remove buildessential from gcc7 dependencies.  Not needed.

* Add DESTDIR to gcc7 install command.

packages/gcc7.rb

+require 'package'
+
+class Gcc7 < Package
+  description 'The GNU Compiler Collection includes front ends for C, C++, Objective-C, Fortran, Ada, and Go.'
+  homepage 'https://www.gnu.org/software/gcc/'
+  version '7.3.0'
+
+  source_url 'https://github.com/gcc-mirror/gcc/archive/gcc-7_3_0-release.zip'
+
+  source_sha256 'd8cb199f0d98bd83e3dac645126722e44137c24cbe5e87e1113dd4270f7530c6'
+
+  depends_on 'gawk'
+  depends_on 'unzip' => :build
+
+  def self.build
+    system "mkdir", "build"
+    Dir.chdir "build" do
+      ENV["LIBRARY_PATH"] = "#{CREW_LIB_PREFIX}"
+      system "../configure",
+             "-v",
+             "--libdir=#{CREW_LIB_PREFIX}",
+             "--prefix=#{CREW_PREFIX}",
+             "--enable-checking=release",
+             "--enable-languages=c,c++,fortran",
+             "--disable-multilib",
+             "--disable-libmpx",
+             "--program-suffix=-7.3"
+      system "make"
+    end
+  end
+
+  def self.install
+    Dir.chdir "build" do
+      ENV["LIBRARY_PATH"] = "#{CREW_LIB_PREFIX}"
+      system "make",
+             "DESTDIR=#{CREW_DEST_DIR}",
+             "install-strip"
+    end
+  end
+end