[PATCH v2 0/4] make: precompile "git-compat-util.h"

Update our Makefile to make use of the recently introduced precompiled header to make builds faster (on my setup by about 35%).

Changes since v1:
  - Update the commit message of patch 3/4 per Junio's suggestion.

SZEDER Gábor (4):
  Makefile: remove XDIFF_OBJS initialization
  cmake: remove any "$(*_OBJS)" variables when parsing Makefile for
    sources
  Makefile: reintroduce REFTABLE_OBJS
  Makefile: precompile "git-compat-util.h"

 .gitignore                          |  1 +
 Makefile                            | 59 +++++++++++++++++++----------
 contrib/buildsystems/CMakeLists.txt |  6 ++-
 3 files changed, 45 insertions(+), 21 deletions(-)

Range-diff against v1:
1:  d52a1476d1 = 1:  d52a1476d1 Makefile: remove XDIFF_OBJS initialization
2:  5d60181efd = 2:  5d60181efd cmake: remove any "$(*_OBJS)" variables when parsing Makefile for sources
3:  0296c6cffc ! 3:  cfc77c0b2c Makefile: reintroduce REFTABLE_OBJS
    @@ Metadata
      ## Commit message ##
         Makefile: reintroduce REFTABLE_OBJS
     
    -    Object files under "reftable/" used to be listed in the REFTABLE_OBJS
    -    Makefile variable so we could build a static library from them.  This
    -    static library was removed in f3b4c89d59 (make: delete REFTABLE_LIB,
    -    add reftable to LIB_OBJS, 2025-10-02), along with filling
    -    REFTALBE_OBJS with object files.
    +    In the next commit we are about to precompile "git-compat-util.h" with
    +    "make" to reduce build times.  But using the precompiled header should
    +    not change what actually gets compiled, therefore a source file can
    +    only be compiled using the precompiled header if the first included
    +    header file is "git-compat-util.h".
     
    -    However, the reftable source files are kind of special, because the
    -    reftable implementation is supposed to be easily includable in other
    -    projects.  Therefore, the reftable source files don't include
    +    The reftable source files are kind of special, because the reftable
    +    implementation is supposed to be easily includable in other projects.
    +    Therefore, the reftable source files don't include
         "git-compat-util.h", with the sole exception of the purposefully
         project-specific "reftable/system.c".  Consequently, they shouldn't be
    -    compiled with our precompiled header, as it does include
    -    "git-compat-util.h".
    +    compiled with our precompiled header.
    +
    +    List object files under "reftable" in the REFTABLE_OBJS Makefile
    +    variable, so in the next commit we'll be able to easily filter them
    +    out and keep building them the old way, without the precompiled
    +    header.
     
    -    Resurrect listing object files under "reftable/" in REFTABLE_OBJS (but
    -    not the static library), so in the next commit we'll be able to easily
    -    filter them out and keep building them the old way, without the
    -    precompiled header.
    +    Note that object files under "reftable/" used to be listed in
    +    REFTABLE_OBJS so we could build a static library from them.  This
    +    static library was removed in f3b4c89d59 (make: delete REFTABLE_LIB,
    +    add reftable to LIB_OBJS, 2025-10-02), along with filling
    +    REFTALBE_OBJS with object files.  This change essentially reverts the
    +    removal of REFTABLE_OBJS, but not the building of that static library.
     
         Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
     
4:  ce9fd91413 = 4:  a933be0664 Makefile: precompile "git-compat-util.h"