build: include directory-local automake files

Since we're using recursive automake, let's use it to make the
configuration a bit easier: by including a `local.am` file for each
directory in our file hierarchy, we can reduce the amount of complexity
in each file.

Makefile.am

@@ -1,15 +1,8 @@
 NULL =
-# Program and sources
-bin_PROGRAMS = jitters
-jitters_SOURCES = \
-    src/jitters.c \
-    $(NULL)
-
-# Add warnings
-jitters_CFLAGS = $(WARN_CFLAGS)
-jitters_LDFLAGS = $(WARN_LDFLAGS)
 
 # Extra files to include in 'make dist'
 EXTRA_DIST = \
     bootstrap \
     $(NULL)
+
+include %D%/src/local.am

src/local.am

@@ -0,0 +1,9 @@
+# Program and sources
+bin_PROGRAMS = jitters
+jitters_SOURCES = \
+    %D%/jitters.c \
+    $(NULL)
+
+# Add warnings
+jitters_CFLAGS = $(WARN_CFLAGS)
+jitters_LDFLAGS = $(WARN_LDFLAGS)