- Add flag --gem-gem to specify the path to the 'gem' tool

  Default is 'gem' and uses $PATH
  #85

lib/fpm/source/gem.rb

@@ -7,6 +7,8 @@
 
 class FPM::Source::Gem < FPM::Source
   def self.flags(opts, settings)
+    settings.source[:gem] = "gem"
+
     opts.on("--bin-path DIRECTORY",
             "The directory to install gem executables") do |path|
       settings.source[:bin_path] = path
@@ -15,6 +17,12 @@ def self.flags(opts, settings)
             "Prefix for gem packages") do |package_prefix|
       settings.source[:package_prefix] = package_prefix
     end
+
+    opts.on("--gem PATH_TO_GEM",
+            "The path to the 'gem' tool (defaults to 'gem' and searches " \
+            "your $PATH)") do |path|
+      settings.source[:gem] = path
+    end
   end # def flags
 
   def get_source(params)
@@ -131,7 +139,7 @@ def make_tarball!(tar_path, builddir)
     end
 
     ::FileUtils.mkdir_p(installdir)
-    args = ["gem", "install", "--quiet", "--no-ri", "--no-rdoc",
+    args = [self[:settings][:gem], "install", "--quiet", "--no-ri", "--no-rdoc",
        "--install-dir", installdir, "--ignore-dependencies", "-E"]
     if self[:settings][:bin_path]
       tmp_bin_path = File.join(tmpdir, self[:settings][:bin_path])