Makefile: clarify definition of TEST_OBJS

Makefile: clarify definition of TEST_OBJS

The definition of TEST_OBJS in commit daa99a91 (Makefile: make sure
test helpers are rebuilt when headers change, 2010-01-26) moved a use
of $X to before the platform-specific section where it gets defined.
There are at least two ways to fix that:

 - Change the definition of TEST_OBJS to use the = delayed
   evaluation operator.  This way, one need not worry about $(X)
   needing to be defined before TEST_OBJS is set.

 - Move the definition of TEST_OBJS to below the definition of $X.

Carry out the second.  The later site of definition makes the code more
readable, since now a reader only has to look down one line to see what
TEST_OBJS is meant to be used for.

Oddly enough, with or without this change the behavior of the Makefile
is the same.  Since TEST_PROGRAMS is defined with delayed evaluation,
the value of

 TEST_OBJS := $(patsubst test-%$X,test-%.o,$(TEST_PROGRAMS))

is independent of the value of $X when it is evaluated: the $X in the
pattern and the $X in $(TEST_PROGRAMS) will simply always cancel out.
Make sure $X has the expected expansion anyway to make the code and
the reader’s sanity more robust in the face of future changes.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>