Makefile: say the default target upfront.

Alex Riesen wants to keep extra makefile targets in config.mak, but
the file is included before any of our real targets.  Having this
at the beginning allows you to do so.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Makefile

+# The default target of this Makefile is...
+all:
+
 # Define MOZILLA_SHA1 environment variable when running make to make use of
 # a bundled SHA1 routine coming from Mozilla. It is GPL'd and should be fast
 # on non-x86 architectures (e.g. PowerPC), while the OpenSSL version (default