configure: --config-mingw32: clarify values, simplify code

Elsewhere at configure the value of $mingw32 is expected to be
exactly either yes or no (not empty, not caps, etc), and while it
complies internally, it also accepts arbitrary override value.

Clarify acceptable values, and simplify the parsing.

configure

@@ -163,7 +163,9 @@ for opt do
   ;;
   --tcc-switches=*) assign_opt "$opt" tcc_switches
   ;;
-  --config-mingw32*) mingw32=$(echo "$opt=yes" | cut -d '=' -f 2)
+  --config-mingw32) mingw32=yes
+  ;;
+  --config-mingw32=*) assign_opt "$opt" mingw32
   ;;
   --config-*) confvars_set "${opt#--config-}"; suggest="no"
   ;;
@@ -217,7 +219,7 @@ Advanced options (experts only):
   --tcc-switches=...       specify implicit switches passed to tcc
 
   --config-uClibc,-musl    enable system specific configurations
-  --config-mingw32         build on windows using msys, busybox, etc.
+  --config-mingw32[=yes|no] build on windows using msys, busybox, etc.
   --config-backtrace=no    disable stack backtraces (with -run or -bt)
   --config-bcheck=no       disable bounds checker (-b)
   --config-predefs=no      do not compile tccdefs.h, instead just include