[ruby-core:114250] [Ruby master Bug#19778] mkmf.rb pkg_config() interaction with RbConfig::CONFIG["cflags"]

Issue #19778 has been reported by rhenium (Kazuki Yamaguchi).

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Bug #19778: mkmf.rb pkg_config() interaction with RbConfig::CONFIG["cflags"]
https://bugs.ruby-lang.org/issues/19778

* Author: rhenium (Kazuki Yamaguchi)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.3.0dev (2023-07-21T09:38:29Z master 22f9735587) [x86_64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
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(This was first reported at https://github.com/ruby/openssl/issues/650: The extension's build breaks in a strange way if `RbConfig::CONFIG["*flags"]` contains the path of an OpenSSL installation but `pkg-config` returns the path of a different OpenSSL installation)

Commit commit:097c3e9cbbf23718371f08c24b2d2297b039f63f ("mkmf.rb: -I flags to $INCFLAGS", Ruby 2.2) changed how mkmf's `pkg_config()` handles the result of the `pkg-config` command. It now stores the `-I` flags in $INCFLAGS and others in $CFLAGS.

mkmf generates a Makefile that compiles source files with:

	$(CC) $(INCFLAGS) $(CPPFLAGS) $(CFLAGS)

and link the final library with:

	$(LDSHARED) -o $@ $(OBJS) $(LIBPATH) $(DLDFLAGS) $(LOCAL_LIBS) $(LIBS)

This "new" behavior of `pkg_config()` is problematic when `RbConfig::CONFIG["{C,CPP}FLAGS"]` also provide `-I` flags and `RbConfig::CONFIG["LDFLAGS"]` provides the matching `-L` flags -- for example, if Ruby is compiled with `./configure --with-opt-dir=<dir>`. This would end up with compiling sources with

	[...] -I<from pkg-config> -I<from RbConfig> [...]

and then linking with

	[...] -L<from RbConfig> -L<from pkg-config> [...]

This doesn't seem right. I don't know which should come earlier, but the order should be consistent. The commit in question clearly describes the change in the commit message, but it doesn't have a linked issue. What is it intended for? Also, what is $INCFLAGS?

On the other hand, `dir_config()` would prepend `-I` flags to $CPPFLAGS and `-L` flags to $LIBPATH, so it doesn't have issues with flags from `RbConfig`, albeit in a differently way.




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