Fix Autotools-generated `libzmq.pc` file #1161

Job	Run time
cron	0s
build (x64, release, windows-2019, ON, ON, Visual Studio 16 2019, v142, vs2019, v142-x64, ON)	5m 23s
build (ubuntu-latest, default, asciidoctor, disabled, select)	2m 31s
build (ubuntu-latest, default, disabled, poll)	2m 27s
build (ubuntu-latest, android, disabled)	3m 23s
build (ubuntu-latest, coverage, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev lcov, enabled,...	3m 9s
build (ubuntu-latest, valgrind, valgrind libgnutls28-dev, enabled)	9m 57s
build (ubuntu-latest, cmake, libsodium, enabled, cmake libsodium-dev, enabled)	3m 28s
build (ubuntu-latest, cmake, enabled, cmake clang-format-11, 1)	39s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	2m 54s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 51s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	2m 43s
build (ubuntu-latest, abi-compliance-checker, abi-dumper abi-compliance-checker, disabled)	3m 3s
build (ubuntu-latest, cmake, clang-tidy clang-tools, enabled, clang++)	3m 34s
build (macos-latest, default, automake autoconf, enabled)	4m 41s
build (macos-latest, default, automake autoconf libsodium, libsodium, disabled)	4m 59s
	56m 42s

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