Fixes Homebrew/homebrew#30 -- GCC can't create executables

The fix is to error out if GCC isn't installed, and to ensure we aren't
setting CC and CXX to something that doesn't exist.

Library/Homebrew/brewkit.rb

@@ -37,7 +37,7 @@ require 'hardware'
 MACOS_VERSION=$1.to_f
 ENV['MACOSX_DEPLOYMENT_TARGET']=$1
 
-# to be consistent with cflags, we ignore the existing environment
+# ignore existing build vars, thus we should have less bugs to deal with
 ENV['LDFLAGS']=""
 
 cflags=%w[-O3]
@@ -63,9 +63,12 @@ else
     cflags<<"-march=prescott"
   end
   cflags<<"-mfpmath=sse"
-  # gcc 4.0 is the default on Leopard
-  ENV['CC']="gcc-4.2"
-  ENV['CXX']="g++-4.2"
+  
+  # use gcc 4.2 if available (Xcode 3.1 and above)
+  if system "which -s gcc-4.2" and $?.success?
+    ENV['CC']="gcc-4.2"
+    ENV['CXX']="g++-4.2"
+  end
 end
 
 cflags<<"-mmmx"

bin/brew

@@ -75,13 +75,18 @@ begin
         exit 0
       end
 
+      require 'brewkit'
+
       if ARGV.verbose?
-        require 'brewkit'
         ohai "Build Environment"
         %w[CFLAGS LDFLAGS CPPFLAGS MAKEFLAGS CC CXX].each do |f|
           puts "#{f}: #{ENV[f]}" unless ENV[f].to_s.empty?
         end
       end
+      
+      unless system "which #{ENV['CC'] or 'cc'} &> /dev/null" and $?.success?
+        raise "We cannot find a c compiler, have you installed the latest Xcode?"
+      end
 
       # we need to ensure a pristine ENV for each process or the formula
       # will start with the ENV from the previous build