Changed WM_OSDEP to use Conditional instead of Substitution

The automake documentation states that using substitution inside the
list of SOURCES will not work and calls for not doing it. The use
of 'EXTRA_xxx' made things look like they worked but is probably not
enough for corner cases.

This patches switches to the conditional method which will be safe.

configure.ac

@@ -101,7 +101,10 @@ AS_CASE(["$host"],
     [*-apple-darwin*],               [WM_OSDEP="darwin"],
     [*-*-solaris*],                  [WM_OSDEP="stub"],  dnl  solaris.c when done
     [WM_OSDEP="stub"])
-AC_SUBST(WM_OSDEP)
+AM_CONDITIONAL([WM_OSDEP_LINUX],   [test "x$WM_OSDEP" = "xlinux"])
+AM_CONDITIONAL([WM_OSDEP_BSD],     [test "x$WM_OSDEP" = "xbsd"])
+AM_CONDITIONAL([WM_OSDEP_DARWIN],  [test "x$WM_OSDEP" = "xdarwin"])
+AM_CONDITIONAL([WM_OSDEP_GENERIC], [test "x$WM_OSDEP" = "xstub"])
 
 
 dnl the prefix

src/Makefile.am

@@ -49,7 +49,6 @@ wmaker_SOURCES = 	\
 	menu.h \
 	misc.c \
 	misc.h \
-	osdep_@WM_OSDEP@.c \
 	monitor.c \
 	motif.c \
 	motif.h \
@@ -105,7 +104,18 @@ wmaker_SOURCES = 	\
 	workspace.c \
 	workspace.h
 
-EXTRA_wmaker_SOURCES = osdep_bsd.c osdep_darwin.c osdep_linux.c osdep_stub.c
+if WM_OSDEP_BSD
+wmaker_SOURCES += osdep_bsd.c
+endif
+if WM_OSDEP_DARWIN
+wmaker_SOURCES += osdep_darwin.c
+endif
+if WM_OSDEP_LINUX
+wmaker_SOURCES += osdep_linux.c
+endif
+if WM_OSDEP_GENERIC
+wmaker_SOURCES += osdep_stub.c
+endif
 
 AM_CFLAGS =