Makefile(s): avoid recipe prefix in conditional statements

In GNU Make commit 07fcee35 ([SV 64815] Recipe lines cannot contain
conditional statements, 2023-05-22) and following, conditional
statements may no longer be preceded by a tab character (which Make
refers to as the recipe prefix).

There are a handful of spots in our various Makefile(s) which will break
in a future release of Make containing 07fcee35. For instance, trying to
compile the pre-image of this patch with the tip of make.git results in
the following:

$ make -v | head -1 && make
GNU Make 4.4.90
config.mak.uname:842: *** missing 'endif'. Stop.

The kernel addressed this issue in 82175d1f9430 (kbuild: Replace tabs
with spaces when followed by conditionals, 2024-01-28). Address the
issues in Git's tree by applying the same strategy.

When a conditional word (ifeq, ifneq, ifdef, etc.) is preceded by one or
more tab characters, replace each tab character with 8 space characters
with the following:

find . -type f -not -path './.git/*' -name Makefile -or -name '*.mak' |
xargs perl -i -pe '
s/(\t+)(ifn?eq|ifn?def|else|endif)/" " x (length($1) * 8) . $2/ge unless /\\$/
'

The "unless /\\$/" removes any false-positives (like "\telse \"
appearing within a shell script as part of a recipe).

After doing so, Git compiles on newer versions of Make:

$ make -v | head -1 && make
GNU Make 4.4.90
GIT_VERSION = 2.44.0.414.gfac1dc44ca9
[...]

$ echo $?
0

Reported-by: Dario Gjorgjevski <dario.gjorgjevski@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>