[ruby-core:88487] [Ruby trunk Bug#14992] mkmf sets DYLD_LIBRARY_PATH on macOS which breaks pkg-config

Issue #14992 has been reported by stevecheckoway (Stephen Checkoway).

----------------------------------------
Bug #14992: mkmf sets DYLD_LIBRARY_PATH on macOS which breaks pkg-config
https://bugs.ruby-lang.org/issues/14992

* Author: stevecheckoway (Stephen Checkoway)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
When `mkmf` runs `pkg-config`, it sets `DYLD_LIBRARY_PATH` on macOS which causes `pkg-config` to not work. As an example, running

~~~
$ ruby2.5 -r mkmf -e 'pkg_config("libxml-2.0")'
~~~

writes the following to `mkmf.log`.

~~~
"pkg-config --exists libxml-2.0"
dyld: Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
  Expected in: /opt/local/lib/libJPEG.dylib
 in /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
package configuration for libxml-2.0 is not found
~~~

The same behavior results from running `pkg-config` with `DYLD_LIBRARY_PATH` set.

~~~
$ DYLD_LIBRARY_PATH=.:/opt/local/lib pkg-config --exists libxml-2.0
dyld: Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
  Expected in: /opt/local/lib/libJPEG.dylib
 in /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
Abort trap: 6
~~~

I verified that `mkmf` is responsible for setting this by writing the following `pkg-config.c` file, compiling it as `pkg-config`, and then running the `ruby2.5` command above with `.` at the front of my `PATH`.

`pkg-config.c`:
~~~ c
#include <stdio.h>
#include <stdlib.h>

int main() {
	printf("DYLD_LIBRARY_PATH=%s\n", getenv("DYLD_LIBRARY_PATH"));
	return 1;
}
~~~

~~~
$ ./pkg-config
DYLD_LIBRARY_PATH=(null)
$ PATH=.:$PATH ruby2.5 -r mkmf -e 'pkg_config("libxml-2.0")'
$ cat mkmf.log
"pkg-config --exists libxml-2.0"
DYLD_LIBRARY_PATH=.:/opt/local/lib
package configuration for libxml-2.0 is not found
~~~

I don't know why `DYLD_LIBRARY_PATH` is being set, but I don't believe it should be. It's not respected by system binaries on macOS and macOS has supported methods of embedding run-time paths in binaries, executables, and bundles that probably should be used instead.



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