Introduce generic variant targets

Change Makefiles to allow multiple binary targets specified by VARIANT
environment variabel. For specified variants a matching main-<VARIANT>.c
source file must be present.

src/util.c

@@ -0,0 +1,42 @@
+/*
+ * =======================================================================================
+ *
+ *   Author:   Jan Eitzinger (je), jan.eitzinger@fau.de
+ *   Copyright (c) 2020 RRZE, University Erlangen-Nuremberg
+ *
+ *   This file is part of MD-Bench.
+ *
+ *   MD-Bench is free software: you can redistribute it and/or modify it
+ *   under the terms of the GNU Lesser General Public License as published
+ *   by the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   MD-Bench is distributed in the hope that it will be useful, but WITHOUT ANY
+ *   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ *   PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
+ *   details.
+ *
+ *   You should have received a copy of the GNU Lesser General Public License along
+ *   with MD-Bench.  If not, see <https://www.gnu.org/licenses/>.
+ * =======================================================================================
+ */
+#include <util.h>
+
+/* Park/Miller RNG w/out MASKING, so as to be like f90s version */
+#define IA 16807
+#define IM 2147483647
+#define AM (1.0/IM)
+#define IQ 127773
+#define IR 2836
+#define MASK 123459876
+
+double myrandom(int* idum)
+{
+    int k= (*idum) / IQ;
+    double ans;
+
+    *idum = IA * (*idum - k * IQ) - IR * k;
+    if(*idum < 0) *idum += IM;
+    ans = AM * (*idum);
+    return ans;
+}