I created a new envvar named INCLUDE_DIR with value C:\MinGW\include, clicked Set now, closed the window and then checked with Process Explorer and found INCLUDE_DIR was added but without a value.
I tried again and the envvar was listed with its value. I clicked Set now again, closed the window and now the value was set.
Created another envvar named LIB_DIR with value C:\MinGW\lib, clicked Set now and closed the window. Now Process Explorer shows INCLUDE_DIR and LIB_DIR empty. I tried again clicking on INCLUDE_DIR first and then on Set now, but now both INCLUDE_DIR and LIB_DIR have C:\MinGW\include as value. I tried again clicking on LIB_DIR and then Set now, but now both have C:\MinGW\lib as value.
Bugs?