refs #1104:
 * fixed compilation on MinGW
 * renamed pre-processor macro Modelica_Export to MODELICA_EXPORT

Compiler tests on Windows with
 * MinGW (GCC 4.4.0 and GCC 4.7.2)
 * Open WATCOM 1.3
 * LCC 2.4.1
 * Borland C/C++ (free command line tools) 5.5
 * Microsoft compilers (VC6 and ≥ VS2005 (Win32 and x64))

Compiler test on CentOS 5.8 x86_64
 * GCC 4.1.2 20080704 (Red Hat 4.1.2-52)

git-svn-id: https://svn.modelica.org/projects/Modelica/trunk@6481 7ce873d0-865f-4ce7-a662-4bb36ea78beb