gcc -DHAVE_CONFIG_H -I. -I.. -I../common -DLOCALEDIR=\"/usr/share/locale\"
-DGNUPG_BINDIR="\"/usr/bin\"" -DGNUPG_LIBEXECDIR="\"/usr/libexec\""
-DGNUPG_LIBDIR="\"/usr/lib64/gnupg\"" -DGNUPG_DATADIR="\"/usr/share/gnupg\""
-DGNUPG_SYSCONFDIR="\"/etc/gnupg\"" -DGNUPG_LOCALSTATEDIR="\"/var\""
-DWITHOUT_NPTH=1 -I/usr/include/p11-kit-1 -O3 -Wall -Wcast-align -Wshadow
-Wstrict-prototypes -Wformat -Wno-format-y2k -Wformat-security -W
-Wno-sign-compare -Wno-missing-field-initializers -Wdeclaration-after-statement
-Wno-pointer-sign -Wpointer-arith -O2 -g -pipe -Wall -Werror=format-security
-Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong
--param=ssp-buffer-size=4 -grecord-gcc-switches
-specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -m64 -mtune=generic -c -o
t_http-t-http.o test -f 't-http.c' || echo './'t-http.c
t-http.c:39:20: fatal error: assuan.h: No such file or directory
#include <assuan.h>

                    ^

looks like configure.ac is just checking if lib exists, nothing more.

There is actually m4 macro for doing things:
dnl AM_PATH_LIBASSUAN([MINIMUM-VERSION,
dnl [ACTION-IF-FOUND [, ACTION-IF-NOT-FOUND ]]])
dnl Test for libassuan and define LIBASSUAN_CFLAGS and LIBASSUAN_LIBS
dnl