Sorry for the delay guys. The notification email went to spam

Ok clearly something changed upstream and it needs sparse now. I'll do the patches

This still isn't fixed? Please add sparse to makedepends

I got the same issue with @sirocco and some others.
Firstly, you need sparse as a dependency package.
Then, it returns that compiler unable to open 'stddef.h'
I fix the issue by adding this patch to mmc-utils
How?:
Paste the text below to create a txt file named "0001-fix.patch" under src/mmc-utils dir, then run "git apply 0001-fix.patch --ignore-whitespace" in console

diff --git a/Makefile b/Makefile
index 06ae0f7..9dc9ff3 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,8 @@ objects = \
    3rdparty/hmac_sha/hmac_sha2.o \
    3rdparty/hmac_sha/sha2.o

-CHECKFLAGS = -Wall -Werror -Wuninitialized -Wundef
+GCC_INCLUDE_DIR := $(shell $(CC) -print-file-name=include)
+CHECKFLAGS = -I$(GCC_INCLUDE_DIR) -Wall -Wuninitialized -Wundef

 DEPFLAGS = -Wp,-MMD,$(@D)/.$(@F).d,-MT,$@

-- 

Then, the compilation should pass.

with sparse

==> Starting build()...
sparse -Wall -Werror -Wuninitialized -Wundef -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -DVERSION=\""523d8f"\" mmc.c
mmc.c: note: in included file (through /usr/include/bits/libc-header-start.h, /usr/include/stdio.h):
/usr/include/features.h:422:4: warning: _FORTIFY_SOURCE requires compiling with optimization (-O)
mmc.c: note: in included file:
/usr/include/stdio.h:34:11: error: unable to open 'stddef.h'
make: *** [Makefile:38: mmc.o] Error 1
==> ERROR: A failure occurred in build().
    Aborting...

I believe sparse is missing from makedepends, the build is failing here:

sparse -Wall -Werror -Wuninitialized -Wundef -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -DVERSION=\""123fd8"\" 3rdparty/hmac_sha/sha2.c
/bin/sh: line 1: sparse: command not found
(...)

I get the same error reported by @paulhart.

I tried running make directly and got a successful build.

Came across https://wiki.archlinux.org/title/makepkg#Makepkg_fails,_but_make_succeeds and set the !buildflags option, which enabled me to successfully build the package

diff --git a/PKGBUILD b/PKGBUILD
index 6e77560..eaa3884 100644
--- a/PKGBUILD
+++ b/PKGBUILD
@@ -15,6 +15,7 @@ provides=("${_pkgname}")
 conflicts=("${_pkgname}")
 source=('git+https://git.kernel.org/pub/scm/utils/mmc/mmc-utils.git')
 sha256sums=('SKIP')
+options=(!buildflags)

 pkgver() {
    cd "${_pkgname}"

This means that one/some of the following flags from my /etc/makepkg.conf caused the build failure

#CPPFLAGS=""
CFLAGS="-march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions \
        -Wp,-D_FORTIFY_SOURCE=3 -Wformat -Werror=format-security \
        -fstack-clash-protection -fcf-protection \
        -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer"
CXXFLAGS="$CFLAGS -Wp,-D_GLIBCXX_ASSERTIONS"
LDFLAGS="-Wl,-O1 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now \
         -Wl,-z,pack-relative-relocs"

@Teeed I'll look into it thanks

Hi,

tried to build this package from 3c6b96c1b06f, but it failed on packing step

==> Starting package()...
install -m755 -d /home/mmc-utils-git/pkg/mmc-utils-git/usr/bin
install mmc /home/mmc-utils-git/pkg/mmc-utils-git/usr/bin
install -m 644 mmc.1 /home/mmc-utils-git/pkg/mmc-utils-git/usr/share/man/man1
install: cannot create regular file '/home/mmc-utils-git/pkg/mmc-utils-git/usr/share/man/man1': No such file or directory
make: *** [Makefile:54: install] Error 1

so here is a patch to make it work:

diff --git a/PKGBUILD b/PKGBUILD
index 40c7b2a..6e77560 100644
--- a/PKGBUILD
+++ b/PKGBUILD
@@ -3,8 +3,8 @@

 _pkgname=mmc-utils
 pkgname=${_pkgname}-git
-pkgver=r113.9582278
-pkgrel=2
+pkgver=r128.f757f41
+pkgrel=1
 epoch=1
 pkgdesc="Userspace tools for MMC/SD devices"
 arch=('x86_64' 'i686' 'aarch64' 'armv7h' 'armv6h')
@@ -28,6 +28,6 @@ build() {

 package() {
        cd "${_pkgname}"
+       mkdir -p "${pkgdir}/usr/share/man/man1/"
        make DESTDIR="${pkgdir}" prefix=/usr install
-       install -Dm644 man/mmc.1 "${pkgdir}/usr/share/man/man1/mmc.1"
 }

I'm getting a compile failure (it would appear the VERSION value in the Makefile isn't being passed to the compiler?):

==> Updated version: mmc-utils-git 1:r109.145c74a-1
==> Starting build()...
make: Entering directory '/home/paulhart/aur/mmc-utils-git/src/mmc-utils'
cc  -Wall -Werror -Wuninitialized -Wundef -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -fexceptions         -Wp,-D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security         -fstack-clash-protection -fcf-protection -Wp,-MMD,./.mmc.o.d,-MT,mmc.o -c mmc.c -o mmc.o
mmc.c: In function ‘help’:
mmc.c:320:26: error: ‘VERSION’ undeclared (first use in this function)
  320 |         printf("\n%s\n", VERSION);
      |                          ^~~~~~~
mmc.c:320:26: note: each undeclared identifier is reported only once for each function it appears in
mmc.c: In function ‘parse_args’:
mmc.c:501:34: error: ‘VERSION’ undeclared (first use in this function)
  501 |                 printf("\n%s\n", VERSION);
      |                                  ^~~~~~~
make: *** [Makefile:37: mmc.o] Error 1
make: Leaving directory '/home/paulhart/aur/mmc-utils-git/src/mmc-utils'
==> ERROR: A failure occurred in build().
    Aborting...