Originally reported downstream in Gentoo here. I've also posted a patch to gcrypt-devel.
Failure snippet:
libtool: compile: armv6j-unknown-linux-gnueabihf-gcc -DHAVE_CONFIG_H -I. -I/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/cipher -I.. -I../src -I/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/src -I../mpi -I/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/mpi -O2 -pipe -mcpu=arm1176jzf-s -mfpu=vfp -mfloat-abi=hard -fvisibility=hidden -fno-delete-null-pointer-checks -Wall -c /var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/cipher/cipher-gcm.c -fPIC -DPIC -o .libs/cipher-gcm.o
/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/cipher/cipher-gcm.c: In function ‘setupM’:
/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/cipher/cipher-gcm.c:591:12: error: ‘features’ undeclared (first use in this function)
591 | else if (features & HWF_ARM_NEON)
| ^~~~~~~~
/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4/cipher/cipher-gcm.c:591:12: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [Makefile:854: cipher-gcm.lo] Error 1
make[2]: Leaving directory '/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4-.arm/cipher'
make[1]: *** [Makefile:502: all-recursive] Error 1
make[1]: Leaving directory '/var/tmp/portage/dev-libs/libgcrypt-1.9.4/work/libgcrypt-1.9.4-.arm'
make: *** [Makefile:434: all] Error 2
* ERROR: dev-libs/libgcrypt-1.9.4::gentoo failed (compile phase):I've explained the issue in more detail within the patch but the gist is that we define features based on one smaller set of CPU features (not checking for NEON) and then try to reference it later on based on a larger set of CPU features (so it can fail if NEON is set because NEON isn't a prerequisite for defining features).