QWT include style

[traversaro]

As I was building Gazebo with conda-forge provided dependencies, I noticed that the build was failing due to how the qwt headers are included in Gazebo, i.e. :

This style of includes strictly requires that the qwt headers are installed as in include/qwt/qwt.h, while the conda-forge feedstock installs them in include/qwt.h . Initially, I thought it was a bug of the qwt conda-forge package, as I checked all other package managers and they were installing the headers in include/qwt/qwt.h. See in for example:

• apt (https://packages.debian.org/sid/amd64/libqwt-qt5-dev/filelist)
• Homebrew (https://bintray.com/homebrew/bottles/qwt#files)
• fedora (https://src.fedoraproject.org/rpms/qwt/blob/master/f/qwt.spec#_198)
• vcpkg ( https://github.com/microsoft/vcpkg/blob/master/ports/qwt/portfile.cmake#L33 )

In all of these package managers the headers are installed as in include/qwt/qwt.h . However, i checked the official documentation of qwt, and indeed there the suggested stile of inclusion is #include <qwt.h>, not #include <qwt/qwt.h> . I also checked other downstream projects, and all of them were using #include <qwt.h> . For this reason, it probably make sense to provide support for both styles.