Add ipopt v3.12.4 package.

Add package for the nonlinear programming solver IPOPT. This
commit depends on the changes made in GitHub PR #711, since IPOPT
requires the headers that correspond to the fake MPI implementation
in sequential MUMPS.

var/spack/repos/builtin/packages/ipopt/package.py

+from spack import *
+
+class Ipopt(Package):
+    """Ipopt (Interior Point OPTimizer, pronounced eye-pea-Opt) is a
+       software package for large-scale nonlinear optimization."""
+    homepage = "https://projects.coin-or.org/Ipopt"
+    url      = "http://www.coin-or.org/download/source/Ipopt/Ipopt-3.12.4.tgz"
+
+    version('3.12.4', '12a8ecaff8dd90025ddea6c65b49cb03')
+    version('3.12.3', 'c560cbfa9cbf62acf8b485823c255a1b')
+    version('3.12.2', 'ec1e855257d7de09e122c446506fb00d')
+    version('3.12.1', 'ceaf895ce80c77778f2cab68ba9f17f3')
+    version('3.12.0', 'f7dfc3aa106a6711a85214de7595e827')
+
+    depends_on("blas")
+    depends_on("lapack")
+    depends_on("pkg-config")
+    depends_on("mumps+double~mpi") 
+    
+    def install(self, spec, prefix):
+        # Dependency directories
+        blas_dir = spec['blas'].prefix
+        lapack_dir = spec['lapack'].prefix
+        mumps_dir = spec['mumps'].prefix
+
+        # Add directory with fake MPI headers in sequential MUMPS
+        # install to header search path
+        mumps_flags = "-ldmumps -lmumps_common -lpord -lmpiseq"
+        mumps_libcmd = "-L%s " % mumps_dir.lib + mumps_flags
+
+        # By convention, spack links blas & lapack libs to libblas & liblapack
+        blas_lib = "-L%s" % blas_dir.lib + " -lblas"
+        lapack_lib = "-L%s" % lapack_dir.lib + " -llapack"
+        
+        configure_args = [
+            "--prefix=%s" % prefix,
+            "--with-mumps-incdir=%s" % mumps_dir.include,
+            "--with-mumps-lib=%s" % mumps_libcmd,
+            "--enable-shared",
+            "--with-blas-incdir=%s" % blas_dir.include,
+            "--with-blas-lib=%s" % blas_lib,
+            "--with-lapack-incdir=%s" % lapack_dir.include,
+            "--with-lapack-lib=%s" % lapack_lib
+            ]
+        
+        configure(*configure_args)
+
+        # IPOPT does not build correctly in parallel on OS X
+        make(parallel=False)
+        make("test", parallel=False)
+        make("install", parallel=False)