Make internal libevhtp optional.

Useful for packaging, when the target already has a libevhtp package. To
use:
make USE_INTERNAL_LIBEVHTP=0

Change-Id: Ia23c6ad9da67eb58ebfd710aa0a3b402408df6d1
Reviewed-on: https://weave-review.googlesource.com/2461
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/Makefile b/Makefile
index df27b58..9810dd0 100644
--- a/Makefile
+++ b/Makefile
@@ -37,7 +37,7 @@
 	-Wwrite-strings
 
 CFLAGS_Debug := \
-	-O0   \
+	-O0 \
 	-g3
 
 CFLAGS_Release := \