Implement static library compilation flag

src/Makefile

@@ -8,10 +8,12 @@ ifndef BUILDDIR_NPTM
 override BUILDDIR_NPTM=$(BUILDDIR)/libnptm
 endif
 ifndef LIBNPTM
-# choose one of the two following lines, depending on whether a static or dynamic libnptm is wanted
-#override LIBNPTM=$(BUILDDIR_NPTM)/libnptm.a
+ifdef STATIC_NPTM
+override LIBNPTM=$(BUILDDIR_NPTM)/libnptm.a
+else
 override LIBNPTM=$(BUILDDIR_NPTM)/libnptm.so
 endif
+endif
 DOCSDIR=$(SRCDIR)/../doc
 
 all: $(BUILDDIR) $(SUBDIRS)