Makefile: define $(LIB_H) in terms of $(FIND_SOURCE_FILES)

Combine the definitions of $(FIND_SOURCE_FILES) and $(LIB_H) to speed
up the Makefile, as these are the two main expensive $(shell) commands
that we execute unconditionally.

When see what was in $(FOUND_SOURCE_FILES) that wasn't in $(LIB_H) via
the ad-hoc test of:

    $(error $(filter-out $(LIB_H),$(filter %.h,$(ALL_SOURCE_FILES))))
    $(error $(filter-out $(ALL_SOURCE_FILES),$(filter %.h,$(LIB_H))))

We'll get, respectively:

    Makefile:850: *** t/helper/test-tool.h.  Stop.
    Makefile:850: *** .  Stop.

I.e. we only had a discrepancy when it came to
t/helper/test-tool.h. In terms of correctness this was broken before,
but now works:

    $ make t/helper/test-tool.hco
        HDR t/helper/test-tool.h

This speeds things up a lot:

    $ git -c hyperfine.hook.setup= hyperfine -L rev HEAD~1,HEAD~0 -s 'make NO_TCLTK=Y' 'make -j1 NO_TCLTK=Y' --warmup 10 -M 10
    Benchmark 1: make -j1 NO_TCLTK=Y' in 'HEAD~1
      Time (mean ± σ):     159.9 ms ±   6.8 ms    [User: 137.2 ms, System: 28.0 ms]
      Range (min … max):   154.6 ms … 175.9 ms    10 runs

    Benchmark 2: make -j1 NO_TCLTK=Y' in 'HEAD~0
      Time (mean ± σ):     100.0 ms ±   1.3 ms    [User: 84.2 ms, System: 20.2 ms]
      Range (min … max):    98.8 ms … 102.8 ms    10 runs

    Summary
      'make -j1 NO_TCLTK=Y' in 'HEAD~0' ran
        1.60 ± 0.07 times faster than 'make -j1 NO_TCLTK=Y' in 'HEAD~1'

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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