pkg-config support insufficient for certain static linking scenarios

[JackKelly-Bellroy]

Describe the bug
Cabal unconditionally invokes pkg-config --libs when collecting arguments, which is not sufficient for certain static linking scenarios:

cabal/Cabal/src/Distribution/Simple/Configure.hs

Line 1640 in 00a2351

ldflags <- pkgconfig ("--libs" : pkgs)

When statically linking, it should use pkg-config --static --libs instead.

To Reproduce

1. Download hw-kafka-client and change its extra-libraries: rdkafka to pkgconfig-depends: rdkafka.
2. Create a package with build-depends: hw-kafka-client, use a cabal.project file to have the test package use your local hw-kafka-client and attempt to build it using a musl64 cross-toolchain (easiest to use haskell.nix for this). Expect to see unresolved symbol errors of the form:

/nix/store/b07pcb91l2q1iasnqj74bajikd43b714-rdkafka-1.6.0-x86_64-unknown-linux-musl/lib/librdkafka.a(rdkafka_ssl.o):function rd_kafka_ssl_ctx_init: error: undefined reference to 'OpenSSL_version_num'

(The failure can be worked around with extra-libraries: rdkafka ssl crypto z.)

Cause of failure:

• The linker command line mentions -lrdkafka but not -lssl -lcrypto, which are required by objects in librdkafka.a.
• rdkafka.pc correctly lists the -lssl etc in Libs.private.
• Reading between the lines of the pkg-config manual, the Libs.private are only added when statically linking. You can confirm this by running pkg-config --static --libs rdkafka and pkg-config --libs rdkafka and comparing their output.
• Cabal only ever invokes pkg-config --libs as far as I can tell (see link at top of issue), so the musl64 build fails.
  A package that has build-depends: hw-kafka-client will not statically link (using musl64 cross-toolchain from haskell.nix) due to missing -lssl and -lcrypto on the linker command-line.

Expected behavior
Additional flags for static linking should make their way through to the linker command line, and the program should build without errors.

[JackKelly-Bellroy]

example.tar.gz

Attached is a minimal reproduction case. unpack the tarball, cd into example and run the shell script - it will download a copy of hw-kafka-client and change it to use pkgconfig-depends. Then run nix-build and you'll see the linker errors I'm talking about. Make sure to set up the IOHK nix cache first.

[phadej]

See #6935 (comment)