C:\MinGW\bin\..\lib\gcc\mingw32\3.4.2\..\..\..\..\mingw32\bin\ld.exe: cannot find -lc:\mysql\lib\opt\libmysql.a
collect2: ld returned 1 exit status
mingw32-make[1]: *** [..\..\..\..\plugins\sqldrivers\qsqlmysqld.dll] Error 1
mingw32-make[1]: Leaving directory `C:/Qt/4.1.0/src/plugins/sqldrivers/mysql'
mingw32-make: *** [debug-all] Error 2

This is a small HOWTO, explaining on how to convert the MySQL libraries
to something which is usable with MinGW.

1) Go to the MySQL lib directory:

   # cd <path_where_mysql_is_installed>\lib\opt

2) Produce a DEF file:

   # pexports libmysql.dll | sed "s/^_//" > libmysql.def

   *NOTE: pexports and sed come with MinGW utilities and msys.

3) Create the MinGW import library:

   # dlltool -U -d libmysql.def -l libmysql.lib.a

At this moment, a MinGW compatible library libmysql.lib.a is created.
But, the headers are still written this way they are using the M$
calling convention. We need to change this.

4) Open the header <path_where_mysql_is_installed>\include\mysql.h
   and change the following around line 44:

   Change:

   #define STDCALL __stdcall

   into:

   #undef  STDCALL
   #define STDCALL

*NOTE: The following is more a hack instead of a solution.
       Maybe you can come up with a cleaner patch.
       Use some extra #ifdef's or something ...

Now, we will build the Qt MySQL plugin using MinGW.

5) Go to the Qt MySQL plugin source directory:

   # cd <path_where_qt_is_installed>\src\plugins\sqldrivers\mysql

6) Launch qmake to generate the Makefile

   # qmake -o Makefile 
"INCLUDEPATH+=<path_where_mysql_is_installed>\include"
     "LIBS+=-L<path_where_mysql_is_installed>\lib\opt" mysql.pro

7) Build the plugin

   # make


PS: I have tried the above for MySQL 4.1 (4.1.12a) and OpenSource Win32 Qt4.