src/Makefile.am: add .DELETE_ON_ERROR

This prevents targets like tool_hugehelp.c from leaving around
half-constructed files if the rule fails with GNU make.

Reported-by: Rafaël Carré <funman@videolan.org>

src/Makefile.am

@@ -21,6 +21,9 @@
 ###########################################################################
 AUTOMAKE_OPTIONS = foreign nostdinc
 
+# remove targets if the command fails
+.DELETE_ON_ERROR:
+
 # Specify our include paths here, and do it relative to $(top_srcdir) and
 # $(top_builddir), to ensure that these paths which belong to the library
 # being currently built and tested are searched before the library which