Firstly I am Using Qt 4.7.4 and Qt Creator 2.4.0, both are mingw versions. I used qt-win-opensource-4.7.4-mingw.exe as my Qt installer and qt-creator-2.4.0-src.zip as my Qt Creator and MinGW-gcc440_1.zip as the compiler version.
I wont go into the mingw, Qt and Qt Creator install I will assume you have the Qt libs and Qt Creator compiled with mingw from herein. My version of Qwt is Qwt-6.0.1. As an aside my version of Qt is installed in C:\Qt\4.7.4 and my version of the Qt Creator is installed C:\Qt\QtCreator
I will assume that if you have used different install directories than me that you are smart enough to replace my directories that I show here with your own, if not well.....
Steps (this is what I did and it worked for me)
1) Unzip Qwt into a temp directory that does not contain any spaces in the path. For me this was C:\temp
2) I did not edit the qwtbuild.pri file (found in the unzipped qwt directory) and let it do its build as a release and debug (it did not build a debug version of the designer plugin as far as I can tell) as it lets me get both the qwt.dll and the qwtd.dll
3) In the qwtconfig.pri file I commented out the Qwtsvg and QwtMathml lines as I wasn't interested in that (you may be but for all I know)
4) Then I went to start->programs->Qt by Nokia v4.7.4 (MinGW OpenSource)->Qt 4.7.4 Command Prompt and started the Qt command prompt window
5) Then from within the command prompt window I changed directory to C:\temp\qwt-6.0.1 and then typed: qmake qwt.pro
6) Then from within the command prompt window I typed: mingw32-make
7) After a bit it finishes compiling and then you type: mingw32-make install
8) After a bit it finishes and you should now have the following directory in your C drive C:\Qwt-6.0.1
9) Then from within the command prompt window I typed: qmake -set QMAKEFEATURES C:\Qwt-6.0.1\features
10) Now close the command prompt window
11) From within windows go to start->settings->control panel->system then go to the advanced tab and select the environment variable button
12) Now select New and for the variable name you type: QT_PLUGIN_PATH and for the variable value I typed: C:\Qwt-6.0.1\plugins
13) Now from the existing environment variables select the PATH variable and add the following at the end: ;C:\Qwt-6.0.1\lib and then hit OK and close the system window. This one lets the widgets show up in the Qt designer.
14) Almost there , now you need to open the qtvars.bat file which for me is located in C:\Qt\4.3.4\bin for editing. The edits that follow let you see the Qwt widgets in the Qt Creator embedded designer.
15) In the qtvars.bat file I added the following after the other set PATH commands: set PATH=%PATH%;C:\Qwt-6.0.1\lib
16) and after the echo to path commands I added: echo -- adding C:\Qwt-6.0.1\lib to PATH <-I am sure this step is probably not really required it is just letting you know what happened on screen.
17) Save and close the qtvars.bat file
18) WIN! You should now be able to start either the Qt Designer (for me its in the Qt program files dropdown list) and/or the Qt Creator (e.g via the Qt 4.7.4 command prompt: cd c:\Qt\Qtcreator\bin followed by qtcreator.exe) and see the Qwt widgets in the form editors
19) Do not forget to add CONFIG +=qwt into your project PRO file
So now you do not have any qwt dll's scattered about the Qt distribution and all is good in the world. As you can see this really is the same as Uwe's INSTALL instructions (for Windows/mingw) but slightly expanded and with the one addition of editing qtvars.bat. Awesome job on the Qwt library Uwe. Well Done mate.