Add libdl on Unix-like systems.

[QuLogic]

PR Summary

Not linking with libdl causes errors when -z defs is in the linker flags.

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[anntzer]

We (and every python extension module) are already linking against libpython, which itself loads libdl. If that's not sufficient to you I'd argue you need to raise the issue upstream (with distutils)...

[QuLogic]

This is not a general addition; we are calling dl* functions in the TkAgg extension, thus this is making the linkage explicit.

[anntzer]

Why is this different? Does -z defs not look into transitively loaded libraries? Should it not? (Perhaps it should indeed not, I just don't know)

[QuLogic]

Here is what the build looks like with -z defs:

g++ -pthread -shared -Wl,-z,relro -Wl,-z,defs -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -g -Wl,-z,relro -Wl,-z,defs -specs=/usr/lib/rpm/redhat/redhat-hardened-ld -g -flto -fuse-linker-plugin -ffat-lto-objects -flto-partition=none build/temp.linux-x86_64-3.6/src/py_converters.o build/temp.linux-x86_64-3.6/src/_tkagg.o -L/usr/local/lib -L/usr/local/lib64 -L/usr/lib -L/usr/lib64 -L/usr/lib64 -lpython3.6m -o build/lib.linux-x86_64-3.6/matplotlib/backends/_tkagg.cpython-36m-x86_64-linux-gnu.so
build/temp.linux-x86_64-3.6/src/_tkagg.o: In function `_dfunc(void*, char const*)':
/builddir/build/BUILD/matplotlib-2.1.2/src/_tkagg.cpp:371: undefined reference to `dlerror'
/builddir/build/BUILD/matplotlib-2.1.2/src/_tkagg.cpp:372: undefined reference to `dlsym'
/builddir/build/BUILD/matplotlib-2.1.2/src/_tkagg.cpp:374: undefined reference to `dlerror'
build/temp.linux-x86_64-3.6/src/_tkagg.o: In function `load_tkinter_funcs()':
/builddir/build/BUILD/matplotlib-2.1.2/src/_tkagg.cpp:409: undefined reference to `dlopen'
/builddir/build/BUILD/matplotlib-2.1.2/src/_tkagg.cpp:433: undefined reference to `dlopen'
/builddir/build/BUILD/matplotlib-2.1.2/src/_tkagg.cpp:441: undefined reference to `dlclose'
collect2: error: ld returned 1 exit status
error: command 'g++' failed with exit status 1

[anntzer]

OK, looks like that's a fedora thing: https://fedoraproject.org/wiki/UnderstandingDSOLinkChange

[QuLogic]

The default for building shared libraries is to ignore undefined things. From the man page:

The default behaviour is to report errors for any undefined symbols referenced in shared libraries if the linker is being used to create an executable, but to allow them if the linker is being used to create a shared library.

But as you've also found, transitive deps are not searched for, and I believe the --copy-dt-needed-entries was switched quite a while back to disable it. (That page is from Fedora 13, which is almost 7 years ago.)

[anntzer]

seems to be a slightly obfuscated way to say "startswith('linux')"...

[QuLogic]

I had assumed it was needed on BSD, but I think that was wrong now.

[QuLogic]

Fixed.