Use gmake for BSD systems

This PR modifies the make-package script so it runs on systems whose default make executable is BSD Make rather than GNU Make. It checks the os type and decides whether to run gmake or make. This is exactly how opam determines which make executable to use.

I also changed the script to use sh instead of bash because make-package doesn't seem to depend on bash-specific features and sh has a more standard path.

make-package

-#!/bin/bash
+#!/bin/sh
 set -e
 # Helper script to build and/or install just one package out of this repository.
 # Assumes that all the other packages it depends on have been installed already!
 
+# Make sure we get a GNU version of make.
+# This is exactly how opam determines which make executable to use.
+OS=$(uname)
+MAKE="make"
+if [ $OS == "FreeBSD" ] || [ $OS == "OpenBSD" ] ||\
+       [ $OS == "NetBSD" ] || [ $OS == "DragonFly" ]; then
+    MAKE="gmake"
+fi
+
 PROJECT="$1"
 shift
 
@@ -26,7 +35,7 @@ grep -E "^$PROJECT/" _CoqProject >> "$COQFILE"
 "${COQBIN}coq_makefile" -f "$COQFILE" -o "$MAKEFILE"
 
 # Run build
-make -f "$MAKEFILE" "$@"
+$MAKE -f "$MAKEFILE" "$@"
 
 # Cleanup
 rm -f ".$MAKEFILE.d" "$MAKEFILE"*