[Bug 2134517] [NEW] libunwind-xx-dev packages do not provide pc files

[mokabar]

Public bug reported:

the llvm-based libunwind-xx-dev packages do not provide the pkg-config files that the libunwind-dev packages provide.
this breaks downstream -dev packages if their pc files depend on libunwind.

one example for this is gstreamer:
```
qt at localhost:~/work/qt/qtmultimedia_build$ cat /usr/lib/aarch64-linux-gnu/pkgconfig/gstreamer-1.0.pc
prefix=/usr
datadir=${prefix}/share
includedir=${prefix}/include
libdir=${prefix}/lib/aarch64-linux-gnu
libexecdir=${prefix}/lib/aarch64-linux-gnu/gstreamer1.0

exec_prefix=${prefix}
toolsdir=${exec_prefix}/bin
pluginsdir=${libdir}/gstreamer-1.0
girdir=${datadir}/gir-1.0
typelibdir=${libdir}/girepository-1.0
pluginscannerdir=${libexecdir}/gstreamer-1.0

Name: gstreamer-1.0
Description: Streaming media framework
Version: 1.24.2
Requires: glib-2.0 >=  2.64.0, gobject-2.0
Requires.private: gmodule-no-export-2.0, libunwind, libdw
Libs: -L${libdir} -lgstreamer-1.0
Libs.private: -lm -ldl
Cflags: -I${includedir}/gstreamer-1.0
```

this means that installing libunwind-xx-dev (e.g. as dependency when
installing libc++) breaks gstreamer's pkg-config integration:

```
qt at localhost:~/work/qt/qtmultimedia_build$ pkg-config --cflags gstreamer-1.0 
Package libunwind was not found in the pkg-config search path.
Perhaps you should add the directory containing `libunwind.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libunwind', required by 'gstreamer-1.0', not found
```

afaict libunwind-xx-dev does not totally "replace" libunwind-dev
(https://git.launchpad.net/ubuntu/+source/llvm-
toolchain-20/tree/debian/control?h=applied/ubuntu/devel#n860)

** Affects: llvm-toolchain-20 (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/2134517

Title:
  libunwind-xx-dev packages do not provide pc files

Status in llvm-toolchain-20 package in Ubuntu:
  New

Bug description:
  the llvm-based libunwind-xx-dev packages do not provide the pkg-config files that the libunwind-dev packages provide.
  this breaks downstream -dev packages if their pc files depend on libunwind.

  one example for this is gstreamer:
  ```
  qt at localhost:~/work/qt/qtmultimedia_build$ cat /usr/lib/aarch64-linux-gnu/pkgconfig/gstreamer-1.0.pc
  prefix=/usr
  datadir=${prefix}/share
  includedir=${prefix}/include
  libdir=${prefix}/lib/aarch64-linux-gnu
  libexecdir=${prefix}/lib/aarch64-linux-gnu/gstreamer1.0

  exec_prefix=${prefix}
  toolsdir=${exec_prefix}/bin
  pluginsdir=${libdir}/gstreamer-1.0
  girdir=${datadir}/gir-1.0
  typelibdir=${libdir}/girepository-1.0
  pluginscannerdir=${libexecdir}/gstreamer-1.0

  Name: gstreamer-1.0
  Description: Streaming media framework
  Version: 1.24.2
  Requires: glib-2.0 >=  2.64.0, gobject-2.0
  Requires.private: gmodule-no-export-2.0, libunwind, libdw
  Libs: -L${libdir} -lgstreamer-1.0
  Libs.private: -lm -ldl
  Cflags: -I${includedir}/gstreamer-1.0
  ```

  this means that installing libunwind-xx-dev (e.g. as dependency when
  installing libc++) breaks gstreamer's pkg-config integration:

  ```
  qt at localhost:~/work/qt/qtmultimedia_build$ pkg-config --cflags gstreamer-1.0 
  Package libunwind was not found in the pkg-config search path.
  Perhaps you should add the directory containing `libunwind.pc'
  to the PKG_CONFIG_PATH environment variable
  Package 'libunwind', required by 'gstreamer-1.0', not found
  ```

  afaict libunwind-xx-dev does not totally "replace" libunwind-dev
  (https://git.launchpad.net/ubuntu/+source/llvm-
  toolchain-20/tree/debian/control?h=applied/ubuntu/devel#n860)

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