Makefile: fix problems with static linking

Avoid a problem with double-derefence of a variable macro name (i.e.,
a macro with part of its name from an AC_SUBST, such as
```$(foo@BAR@baz)```.

In what might be a bug in Automake 1.14.1, if you do a pattern like
this:

```makefile
lib_LTLIBRARIES = lib@A_PREFIX@a_lib.la
noinst_LTLIBRARIES = lib@A_PREFIX@a_noinst.la

lib@A_PREFIX@a_lib_la_SOURCES = a.c

lib@A_PREFIX@a_noinst_la_SOURCES = $(lib@A_PREFIX@a_lib_la_SOURCES)
```

Then in the resulting Makefile, the value of
```$(lib@A_PREFIX@a_lib_la_OBJECTS)``` will be *blank* (when it really
should be ```a.o```).

To workaround this potential bug, I've simply avoided doing
double-derefences like this, and effectively set the second
```_SOURCES``` line equal to ```a.c``` (just like the first
```_SOURCES``` line).

Fixes #250.