No longer compile with LLVM by default

Justification:
* LLVM is too immature, it often fails to compile, sometimes irreproducibly
* The performance of the resulting bytecode is often slower than the GCC
  equivalent (eg MySQL)

If you want to continue using LLVM you can. Set HOMEBREW_USE_LLVM or brew with
--use-llvm.

We also now use the default compiler for each platform. So GCC 4.2 on Snow and
GCC 4.0 on Leopard. This also means that new formula are more likely to just
work as many over complicated build systems get upset when you mix things up.

I also did a bunch more new research regarding compiler flags. We now set the
right -march for nehalem procs and don't add redundant flags like msse3 (which
is automatically applied for all the -march settings we use).

Reducing the number of cflags will improve the overall reliability of our
platform.

Library/Homebrew/extend/ENV.rb

@@ -29,6 +29,10 @@ module HomebrewEnvExtension
   def setup_build_environment
     # Clear CDPATH to avoid make issues that depend on changing directories
     ENV.delete('CDPATH')
+    ENV.delete('CPPFLAGS')
+    ENV.delete('LDFLAGS')
+    ENV.delete('CC')
+    ENV.delete('CXX')
 
     ENV['MAKEFLAGS']="-j#{Hardware.processor_count}"
 
@@ -38,13 +42,9 @@ module HomebrewEnvExtension
       ENV['LDFLAGS'] = "-L#{HOMEBREW_PREFIX}/lib"
       # CMake ignores the variables above
       ENV['CMAKE_PREFIX_PATH'] = "#{HOMEBREW_PREFIX}"
-    else
-      # ignore existing build vars, thus we should have less bugs to deal with
-      ENV['CPPFLAGS'] = ''
-      ENV['LDFLAGS'] = ''
     end
 
-    if MACOS_VERSION >= 10.6 or ENV['HOMEBREW_USE_LLVM']
+    if MACOS_VERSION >= 10.6 and (ENV['HOMEBREW_USE_LLVM'] or ARGV.include? '--use-llvm')
       # you can install Xcode wherever you like you know.
       prefix = `/usr/bin/xcode-select -print-path`.chomp
       prefix = "/Developer" if prefix.to_s.empty?
@@ -53,46 +53,46 @@ module HomebrewEnvExtension
       ENV['CXX'] = "#{prefix}/usr/bin/llvm-g++"
       cflags = %w{-O4} # link time optimisation baby!
     else
-      ENV['CC']="gcc-4.2"
-      ENV['CXX']="g++-4.2"
       cflags = ['-O3']
     end
+
     # in rare cases this may break your builds, as the tool for some reason wants
     # to use a specific linker, however doing this in general causes formula to
     # build more successfully because we are changing CC and many build systems
     # don't react properly to that
-    ENV['LD']=ENV['CC']
+    ENV['LD'] = ENV['CC'] if ENV['CC']
 
     # optimise all the way to eleven, references:
     # http://en.gentoo-wiki.com/wiki/Safe_Cflags/Intel
     # http://forums.mozillazine.org/viewtopic.php?f=12&t=577299
     # http://gcc.gnu.org/onlinedocs/gcc-4.2.1/gcc/i386-and-x86_002d64-Options.html
+    # we don't set, eg. -msse3 because the march flag does that for us
+    #   http://gcc.gnu.org/onlinedocs/gcc-4.3.3/gcc/i386-and-x86_002d64-Options.html
     if MACOS_VERSION >= 10.6
       case Hardware.intel_family
-      when :penryn, :core2
-        # no need to add -mfpmath it happens automatically with 64 bit compiles
+      when :nehalem, :penryn, :core2
+        # the 64 bit compiler adds -mfpmath=sse for us
         cflags << "-march=core2"
       when :core
         cflags<<"-march=prescott"<<"-mfpmath=sse"
       end
-    else
+      # gcc doesn't auto add msse4 or above (based on march flag) yet
       case Hardware.intel_family
-      when :penryn, :core2
+      when :nehalem
+        cflags << "-msse4" # means msse4.2 and msse4.1
+      when :penryn
+        cflags << "-msse4.1"
+      end
+    else
+      # gcc 4.0 didn't support msse4
+      case Hardware.intel_family
+      when :nehalem, :penryn, :core2
         cflags<<"-march=nocona"
       when :core
         cflags<<"-march=prescott"
       end
       cflags<<"-mfpmath=sse"
     end
-    cflags<<"-mmmx"
-    case Hardware.intel_family
-    when :nehalem
-      cflags<<"-msse4.2"
-    when :penryn
-      cflags<<"-msse4.1"
-    when :core2, :core
-      cflags<<"-msse3"
-    end
 
     ENV['CFLAGS'] = ENV['CXXFLAGS'] = "#{cflags*' '} #{SAFE_CFLAGS_FLAGS}"
   end
@@ -102,6 +102,11 @@ module HomebrewEnvExtension
   end
   alias_method :j1, :deparallelize
 
+  # recommended by Apple, but, eg. wget won't compile with this flag, so…
+  def fast
+    remove_from_cflags /-O./
+    append_to_cflags '-fast'
+  end
   def O3
     # Sometimes O4 just takes fucking forever
     remove_from_cflags /-O./
@@ -119,27 +124,18 @@ module HomebrewEnvExtension
   end
 
   def gcc_4_0_1
-    case MACOS_VERSION
-    when 10.5
-      self['CC']=nil
-      self['CXX']=nil
-      self['LD']=nil
-    when 10.6..11.0
-      self['CC']='gcc-4.0'
-      self['CXX']='g++-4.0'
-      self['LD']=self['CC']
-      remove_from_cflags '-march=core2'
-      self.O3
-    end
-    remove_from_cflags '-msse4.1'
-    remove_from_cflags '-msse4.2'
+    self['CC'] = self['LD'] = '/usr/bin/gcc-4.0'
+    self['CXX'] = '/usr/bin/g++-4.0'
+    self.O3
+    remove_from_cflags '-march=core2'
+    remove_from_cflags %r{-msse4(\.\d)?/}
   end
   alias_method :gcc_4_0, :gcc_4_0_1
 
   def gcc_4_2
     # Sometimes you want to downgrade from LLVM to GCC 4.2
-    self['CC']="gcc-4.2"
-    self['CXX']="g++-4.2"
+    self['CC']="/usr/bin/gcc-4.2"
+    self['CXX']="/usr/bin/g++-4.2"
     self['LD']=self['CC']
     self.O3
   end