ENH: add int64 variant to scotch (as per metis +int64) (#3536)

BUG: provide patch for scotch 6.0.4 if building non-threaded

- corresponds to issue #20179 from the scotch issue trackter

var/spack/repos/builtin/packages/scotch/nonthreaded-6.0.4.patch

@@ -0,0 +1,11 @@
+--- scotch_6.0.4.orig/src/libscotch/common.h	2015-03-01 10:14:02.000000000 +0100
++++ scotch_6.0.4/src/libscotch/common.h	2017-03-27 13:07:18.644221999 +0200
+@@ -306,6 +306,8 @@
+   ThreadLaunchStartFunc     stafptr;              /*+ Pointer to start routine +*/
+   ThreadLaunchJoinFunc      joifptr;              /*+ Pointer to join routine  +*/
+   ThreadBarrier             barrdat;              /*+ Barrier data structure   +*/
++#else
++  int                       thrdnbr;    /* dummy for non-threaded */
+ #endif /* ((defined COMMON_PTHREAD) || (defined SCOTCH_PTHREAD)) */
+ } ThreadGroupHeader;
+ 

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

@@ -31,7 +31,7 @@ class Scotch(Package):
        partitioning, graph clustering, and sparse matrix ordering."""
 
     homepage = "http://www.labri.fr/perso/pelegrin/scotch/"
-    url = "http://gforge.inria.fr/frs/download.php/latestfile/298/scotch_6.0.3.tar.gz"
+    url = "http://gforge.inria.fr/frs/download.php/latestfile/298/scotch_6.0.3.tar.gz"  # noqa: E501
     base_url = "http://gforge.inria.fr/frs/download.php/latestfile/298"
     list_url = "http://gforge.inria.fr/frs/?group_id=248"
 
@@ -50,12 +50,17 @@ class Scotch(Package):
             description='Build a shared version of the library')
     variant('metis', default=True,
             description='Build metis and parmetis wrapper libraries')
+    variant('int64', default=False,
+            description='Use int64_t for SCOTCH_Num typedef')
 
     depends_on('flex@:2.6.1', type='build')
     depends_on('bison', type='build')
     depends_on('mpi', when='+mpi')
     depends_on('zlib', when='+compression')
 
+    # Version-specific patches
+    patch('nonthreaded-6.0.4.patch', when='@6.0.4')
+
     # NOTE: In cross-compiling environment parallel build
     # produces weird linker errors.
     parallel = False
@@ -87,9 +92,13 @@ def configure(self):
             '-DCOMMON_RANDOM_FIXED_SEED',
             '-DSCOTCH_DETERMINISTIC',
             '-DSCOTCH_RENAME',
-            '-DIDXSIZE64'
+            '-DIDXSIZE64',  # SCOTCH_Idx typedef: indices for addressing
         ]
 
+        # SCOTCH_Num typedef: size of integers in arguments
+        if '+int64' in self.spec:
+            cflags.append('-DINTSIZE64')
+
         if self.spec.satisfies('platform=darwin'):
             cflags.extend([
                 '-Drestrict=__restrict'