Initially added in https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=cb196546c62e9894a46900e8078f753388d4cc1a.
Commit mentions it being from Debian but not found any other references yet.

We use -largp fairly extensively across the tree which this patch
accommodates (easier than forcing includes?).
--- a/Makefile.am	2022-05-03 03:41:05.000000000 -0400
+++ b/Makefile.am	2022-05-30 12:28:33.850249380 -0400
@@ -21,19 +21,19 @@
 AUTOMAKE_OPTIONS = foreign
 SUBDIRS = . testsuite
 
-LIBOBJS = @LIBOBJS@
+LTLIBOBJS = @LTLIBOBJS@
 
-noinst_LIBRARIES = libargp.a
+lib_LTLIBRARIES = libargp.la
 noinst_PROGRAMS = argp-test
 noinst_HEADERS = argp.h argp-fmtstream.h argp-namefrob.h # argp-comp.h
 
 EXTRA_DIST = mempcpy.c strchrnul.c strndup.c Versions
 
 # Leaves out argp-fs-xinl.c and argp-xinl.c
-libargp_a_SOURCES = argp-ba.c argp-eexst.c argp-fmtstream.c \
+libargp_la_SOURCES = argp-ba.c argp-eexst.c argp-fmtstream.c \
 		    argp-help.c argp-parse.c argp-pv.c \
 		    argp-pvh.c
 
-libargp_a_LIBADD = $(LIBOBJS)
+libargp_la_LIBADD = $(LTLIBOBJS)
 
-argp_test_LDADD = libargp.a
+argp_test_LDADD = libargp.la
--- a/configure.ac	2022-05-03 03:41:05.000000000 -0400
+++ b/configure.ac	2022-05-30 12:27:08.688735341 -0400
@@ -17,6 +17,7 @@
 AC_PROG_MAKE_SET
 AC_PROG_RANLIB
 AM_PROG_CC_STDC
+AC_PROG_LIBTOOL
 
 if test "x$am_cv_prog_cc_stdc" = xno ; then
   AC_ERROR([the C compiler doesn't handle ANSI-C])
--- a/testsuite/Makefile.am	2022-05-03 03:41:05.000000000 -0400
+++ b/testsuite/Makefile.am	2022-05-30 12:27:08.688735341 -0400
@@ -5,7 +5,7 @@
 
 noinst_PROGRAMS = $(TS_PROGS) ex1 ex3 ex4
 
-LDADD = ../libargp.a
+LDADD = -L../.libs -largp
 
 EXTRA_DIST = $(TS_SH) run-tests
 CLEANFILES = test.out