Workaround for [http://sourceforge.net/p/mingw/bugs/2065/|MinGW bug #2065]. 
Both MinGW and MinGW-w64 (32-bit) are affected. 
Even though Win64 is not affected, adding -static-libgcc doesn't harm there, and we don't want to distrubute additional dll's with MinGW-compiled Tcl anyway.