[PATCH v4 21/23] Makefile: disable GNU make built-in wildcard rules

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Override built-in rules of GNU make that use a wildcard target. This
speeds things up significantly as we don't need to stat() so many
files just in case we'd be able to retrieve their contents from RCS or
SCCS. See [1] for an old mailing list discussion about how to disable
these.

This gives us a noticeable speedup on a no-op run:

    $ git hyperfine -L rev HEAD~1,HEAD~0 -s 'make -j8 all NO_TCLTK=Y' 'make NO_TCLTK=Y' --warmup 10 -M 10
    Benchmark 1: make NO_TCLTK=Y' in 'HEAD~1
      Time (mean ± σ):     182.2 ms ±   4.1 ms    [User: 146.8 ms, System: 49.5 ms]
      Range (min … max):   179.6 ms … 193.4 ms    10 runs

    Benchmark 2: make NO_TCLTK=Y' in 'HEAD~0
      Time (mean ± σ):     167.9 ms ±   1.3 ms    [User: 127.8 ms, System: 55.6 ms]
      Range (min … max):   166.1 ms … 170.4 ms    10 runs

    Summary
      'make NO_TCLTK=Y' in 'HEAD~0' ran
        1.09 ± 0.03 times faster than 'make NO_TCLTK=Y' in 'HEAD~1'

Running the same except with 'strace -c -S calls make' as the
benchmark command shows (under --show-output) that we went from ~7716
syscalls to ~7519, mostly a reduction in [l]stat().

We could also invoke make with "-r" by setting "MAKEFLAGS = -r" early,
adding a "-r" variant to the above benchmark shows that it may be 1.01
or so faster (but in my tests, always with a much bigger error
bar). But doing so is a much bigger hammer, since it will disable all
built-in rules, some (all?) of which can be seen with:

    make -f/dev/null -p | grep -v -e ^# -e ^$

We may have something that relies on them, so let's go for the more
isolated optimization here that gives us most or all of the wins.

1. https://lists.gnu.org/archive/html/help-make/2002-11/msg00063.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 shared.mak | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/shared.mak b/shared.mak
index 2d5d5b2fb9b..20109e6a90e 100644
--- a/shared.mak
+++ b/shared.mak
@@ -1,3 +1,14 @@
+### Remove GNU make implicit rules
+
+## This speeds things up since we don't need to look for and stat() a
+## "foo.c,v" every time a rule referring to "foo.c" is in play. See
+## "make -p -f/dev/null | grep ^%::'".
+%:: %,v
+%:: RCS/%,v
+%:: RCS/%
+%:: s.%
+%:: SCCS/s.%
+
 ### Flags affecting all rules
 
 # A GNU make extension since gmake 3.72 (released in late 1994) to
-- 
2.34.0.796.g2c87ed6146a