Add $KOKA_PATH, and pkg-config lookup for C libraries

[timbertson]

This PR implements the two ideas I mentioned in discord:

• use $KOKA_PATH as an additional list of include directories, similar to many other languages
• add support for resolving C libraries via pkg-config. This is a standard which is supported by a number of build tools (including many conan / vcpkg packages). This may make the existing two integrations redundant one day, but for now I've just added it as an additional source.

Sample output finding libuv with --verbose=4 (the library is provided by nix, thus the long paths):

codegen : uv/utils
generated c: .koka/v3.2.2/clang-debug-5f0454/uv_utils
libPath : libuv.a not found in ["/usr/local/lib","/usr/lib","/lib","/opt/homebrew/lib"]
command : /nix/store/hj7iydwvhkslzsvw82iilv8say0xv5n7-pkg-config-wrapper-0.29.2/bin/pkg-config --variable=libdir libuv
out     : /nix/store/nwck8lyydw82yvxc8gz9h5b92yhwdcy6-libuv-1.50.0/lib
libPath : /nix/store/nwck8lyydw82yvxc8gz9h5b92yhwdcy6-libuv-1.50.0/lib/libuv.a
command : /nix/store/hj7iydwvhkslzsvw82iilv8say0xv5n7-pkg-config-wrapper-0.29.2/bin/pkg-config --variable=includedir libuv
out     : /nix/store/sqbwc4gyayj983i6qkw3bmikxbf86v2c-libuv-1.50.0-dev/include
library : /nix/store/nwck8lyydw82yvxc8gz9h5b92yhwdcy6-libuv-1.50.0/lib/libuv.a
codegen done: uv/utils
link    : uv/utils
command : /nix/store/xpja3ad45p55aryvgng6g9grp8yivvsd-clang-wrapper-19.1.7/bin/clang -Wall -Wextra -Wpointer-arith -Wshadow -Wstrict-aliasing -Wno-unknown-pragmas -Wno-missing-field-initializers -Wno-unused-parameter -Wno-unused-variable -Wno-unused-value -Wno-unused-but-set-variable -Wno-cast-qual -Wno-undef -Wno-reserved-id-macro -Wno-unused-macros -Wno-cast-align -Wno-unknown-warning-option -g -Og -c -I /Users/tcuthbertson/.local/share/koka/v3.2.2/kklib/include -I /nix/store/sqbwc4gyayj983i6qkw3bmikxbf86v2c-libuv-1.50.0-dev/include -DKK_MIMALLOC=8 -o .koka/v3.2.2/clang-debug-5f0454/uv_utils.o .koka/v3.2.2/clang-debug-5f0454/uv_utils.c
building: 0.822507s

[TimWhiting]

It looks like we already have a utility for splitting delimited paths used elsewhere in this file: undelimPaths. Not sure if Daan would want to avoid the extra dependency, and use this, or if we should use a version that might handle slightly more edge cases than we do. We tend to avoid adding more dependencies.

[timbertson]

Thanks, happy to use that - I wasn't aware of it. GHC reported that filepath was a hidden dependency which I assumed meant it was already required transitively, but there's a chance it was only used for tests previously.

[TimWhiting]

Hmm, yes, that might be the case.