LLVM depends on cmake, and does not depend on isl

Add cmake requirement.

Remove gmp and isl requirements. Using an external isl leads to a build failure for me on a fairly standard Fedora Linux workstation. The Spack package file says that isl is required for polly, however, the polly documentation states that as of LLVM 3.7, polly includes isl, and has no external dependencies any more.

var/spack/packages/llvm/package.py

@@ -48,6 +48,9 @@ class Llvm(Package):
     variant('gold', default=True, description="Add support for LTO with the gold linker plugin")
 
 
+    # Build dependency
+    depends_on('cmake @2.8.12.2:')
+
     # Universal dependency
     depends_on('python@2.7:')
 
@@ -60,8 +63,8 @@ class Llvm(Package):
     depends_on('binutils+gold', when='+gold')
 
     # polly plugin
-    depends_on('gmp', when='+polly')
-    depends_on('isl', when='+polly')
+    depends_on('gmp', when='@:3.6.999 +polly')
+    depends_on('isl', when='@:3.6.999 +polly')
 
     base_url =  'http://llvm.org/releases/%%(version)s/%(pkg)s-%%(version)s.src.tar.xz'
     llvm_url = base_url % { 'pkg' : 'llvm'}