I'm not sure, if it is relevant, but
I tried to build the newer version, the v2.1.11, id est the "GnuPG Modern"
and it did not go so well either, despite the fact that
I custom-built the dependent libraries and put their bin folders to
PATH and lib64 folders LD_LIBRARY_PATH prior to attempting
to build the gnupg.

libksba has not been installed properly.

Thank You. I noticed later that, indeed, at the first instance,
there's a problem with the library, but I corrected that issue
with the other try, the one that is described at

https://bugs.gnupg.org/gnupg/file821/2016_04_gnupg_v_2_1_11_build_log.txt

---citation--start----
gcc -DHAVE_CONFIG_H -I. -I.. -I../common -
DLOCALEDIR=\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016_04/gnup
g/share/locale\" -
DGNUPG_BINDIR="\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016_04/
gnupg/bin\"" -
DGNUPG_LIBEXECDIR="\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016
_04/gnupg/lib\"" -
DGNUPG_LIBDIR="\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016_04/
gnupg/lib64/gnupg\"" -
DGNUPG_DATADIR="\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016_04
/gnupg/share/gnupg\"" -
DGNUPG_SYSCONFDIR="\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016
_04/gnupg/etc/gnupg\"" -
DGNUPG_LOCALSTATEDIR="\"/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2
016_04/gnupg/var\"" -
I/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016_04/libgcrypt/includ
e -
I/home/ts2/m_local/bin_p_originaalid/GNU_Privacy_Guard/v2016_04/libksba/include
-Wall -Wno-pointer-sign -Wpointer-arith -mtune=native -ftree-vectorize -MT
libkeybox_a-keybox-util.o -MD -MP -MF .deps/libkeybox_a-keybox-util.Tpo -c -o
libkeybox_a-keybox-util.o test -f 'keybox-util.c' || echo './'keybox-util.c
In file included from keybox-defs.h:42:0,

from keybox-util.c:29:

../common/stringhelp.h: In function ‘make_filename’:
../common/stringhelp.h:55:52: error: expected declaration specifiers before â
€˜GPGRT_ATTR_SENTINEL’
char *make_filename( const char *first_part, ... ) GPGRT_ATTR_SENTINEL(0);

^

---citation--end------

Besides, given the small size of the GnuPG, shouldn't the
few GnuPG specific libraries just be subfolders of the
GnuPG project? If not in the repository, then at least
at the release tar-ball? It would avoid the
"library wrongly installed" part.

Those libraries are not GnuPG specific.