build: use configure rule whether host tool is built

Use a configure variable to control whether the host tool is built.  It
is built by default.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ic5052fbd20cb16fe7fec8fc09d25b90457a5a645
diff --git a/Makefile.am b/Makefile.am
index 9dbe34a..8789663 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -86,4 +86,8 @@
 	$(CODE_COVERAGE_CXXFLAGS) \
 	-flto
 
-SUBDIRS = . test tools
+SUBDIRS = . test
+
+if BUILD_HOST_TOOL
+SUBDIRS += tools
+endif