https://github.com/the-tcpdump-group/libpcap/commit/84cb8cfdbf99a5fbc8127e3f092dc4d36ab513e9

From 84cb8cfdbf99a5fbc8127e3f092dc4d36ab513e9 Mon Sep 17 00:00:00 2001
From: Guy Harris <gharris@sonic.net>
Date: Mon, 2 Aug 2021 01:04:53 -0700
Subject: [PATCH] pcap-config: don't provide -L/usr/lib for pkg-config --libs.

It shouldn't be necessary, as C compilers generally look there by
default and...

...it can cause problems if the libpcap you want *isn't* in /usr/lib.
For example, on some systems that support both 32-bit and 64-bit
executables, this might cause the 32-bit library to be found, even on
64-bit platforms, with hilarity ensuing afterwards.

In particular, on Solaris 11, where /usr/lib has the 32-bit libraries
and /usr/lib/{something} has the 64-bit libraries ({something} depends
on whether it's SPARC or x86), that's what happens if you try to do a
CMake build of tcpdump against the system libpcap:

The CMake file for finding pcap converts the -lpcap provided by
pcap-config into the absolute path of libpcap, and that's the path of
the 32-bit library, as it looks in /usr/lib.  (CMake really wants "find
library" scripts to supply a list of libraries giving their absolute
paths.)

Thus, if you're using GCC, the tests done to find out what pcap APIs are
available will fail, as the test programs get build 64-bit but are
linked with the 32-bit libpcap; the link fails as you're mixing 32-bit
and 64-bit code, and the CMake script treats that as meaning "the
function isn't available".

(Sun C apparently somehow manages either to build 32-bit code by
default, so that linking with /usr/lib/libpcap.so succeeds, or realizes
that linking 64-bit code with /usr/lib/{library}.so is bogus and links
with /usr/lib/{something}/libpcap.so instead.)

Debian removed the -L in pcap-config for similar reasons; to quote the
comment at the beginning of the patch file:

	Remove -L<libdir> from default pcap-config --libs output, as
	libdir is already in the default toolchain search path on
	Debian, and we want the generated script to be arch-independent.

(We don't remove it from the .pc file; we assume that 1) pkg-config and
2) the packager of libpcap does what is necessary to make this work.)
--- a/pcap-config.in
+++ b/pcap-config.in
@@ -41,6 +41,13 @@ do
 	esac
 	shift
 done
+#
+# If libdir isn't /usr/lib, add it to the link-time linker path.
+#
+if [ "$libdir" != "/usr/lib" ]
+then
+	LPATH=-L$libdir
+fi
 if [ "$V_RPATH_OPT" != "" ]
 then
 	#
@@ -59,16 +66,16 @@ then
 	#
 	if [ "$show_cflags" = 1 -a "$show_libs" = 1 ]
 	then
-		echo "-I$includedir -L$libdir -lpcap $LIBS"
+		echo "-I$includedir $LPATH -lpcap $LIBS"
 	elif [ "$show_cflags" = 1 -a "$show_additional_libs" = 1 ]
 	then
-		echo "-I$includedir -L$libdir $LIBS"
+		echo "-I$includedir $LPATH $LIBS"
 	elif [ "$show_cflags" = 1 ]
 	then
 		echo "-I$includedir"
 	elif [ "$show_libs" = 1 ]
 	then
-		echo "-L$libdir -lpcap $LIBS"
+		echo "$LPATH -lpcap $LIBS"
 	elif [ "$show_additional_libs" = 1 ]
 	then
 		echo "$LIBS"
@@ -80,7 +87,7 @@ else
 	#
 	if [ "$show_cflags" = 1 -a "$show_libs" = 1 ]
 	then
-		echo "-I$includedir -L$libdir $RPATH -l$PACKAGE_NAME"
+		echo "-I$includedir $LPATH $RPATH -l$PACKAGE_NAME"
 	elif [ "$show_cflags" = 1 -a "$show_additional_libs" = 1 ]
 	then
 		echo "-I$includedir"
@@ -89,6 +96,6 @@ else
 		echo "-I$includedir"
 	elif [ "$show_libs" = 1 ]
 	then
-		echo "-L$libdir $RPATH -l$PACKAGE_NAME"
+		echo "$LPATH $RPATH -l$PACKAGE_NAME"
 	fi
 fi