[PATCH] Makefile: remove archives before manipulating them with 'ar'

The rules creating the $(LIB_FILE) and $(XDIFF_LIB) archives used to be:

  $(QUIET_AR)$(RM) $@ && $(AR) $(ARFLAGS) $@ $^

until commit 7b76d6bf22 (Makefile: add and use the ".DELETE_ON_ERROR" flag, 2021-06-29) removed the '$(RM) $@' part, claiming that "we can rely on the "c" (create) being present in ARFLAGS", and (I presume) assuming that it means that the named archive is created from scratch.

Unfortunately, that's not what the 'c' flag does, it merely "Suppress the diagnostic message that is written to standard error by default when the archive is created" [1]. Consequently, all object files that are already present in an existing archive and are not replaced will remain there. This leads to linker errors in back-to-back builds of different revisions without a 'make clean' between them if source files going into these archives are renamed in between:

  # The last commit renaming files that go into 'libgit.a':
  # bc62692757 (hash-lookup: rename from sha1-lookup, 2020-12-31)
  #  sha1-lookup.c => hash-lookup.c | 14 +++++++-------
  #  sha1-lookup.h => hash-lookup.h | 12 ++++++------
  $ git checkout bc62692757^
  HEAD is now at 7a7d992d0d sha1-lookup: rename `sha1_pos()` as `hash_pos()`
  $ make
  [...]
  $ git checkout 7b76d6bf22
  HEAD is now at 7b76d6bf22 Makefile: add and use the ".DELETE_ON_ERROR" flag
  $ make
  [...]
      AR libgit.a
      LINK git
  /usr/bin/ld: libgit.a(hash-lookup.o): in function `bsearch_hash':
  /home/szeder/src/git/hash-lookup.c:105: multiple definition of `bsearch_hash'; libgit.a(sha1-lookup.o):/home/szeder/src/git/sha1-lookup.c:105: first defined here
  collect2: error: ld returned 1 exit status
  make: *** [Makefile:2213: git] Error 1

Restore the original make rules to first remove $(LIB_FILE) and $(XDIFF_LIB) and then create them from scratch to avoid these build errors.

[1] Quoting POSIX at:
    https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ar.html

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)