Improve sim win32 cross compiling.

- Extend configure to allow setting the cross compiler prefix using the
  environment variable CROSS. This allows using a compiler that is prefixed
  differently than the prefix configure does assume.
- When searching for sdl-config also check for a prefixed version. Some
  toolchains prefix sdl-config (like mingw-cross-env).

This enables cross compiling the sim with at least the MinGW packages provided
by Fedora (different prefix) and mingw-cross-env (different prefix and prefixes
sdl-config. There's a non-prefixed version as well, but that folder also
includes the non-prefixed compiler, so using the prefixed version is
preferable.)

starting

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@30484 a1c6a512-1295-4272-9138-f99709370657

tools/configure

@@ -147,10 +147,13 @@ findtool(){
 # scan the $PATH for sdl-config - check whether for a (cross-)win32
 # sdl as requested
 findsdl(){
-  file="sdl-config"
+  # sdl-config might (not) be prefixed for cross compiles so try both.
+  files="sdl-config:${CROSS}sdl-config"
   winbuild="$1"
 
   IFS=":"
+  for file in $files
+  do
     for path in $PATH
     do
       #echo "checks for $file in $path" >&2
@@ -168,6 +171,7 @@ findsdl(){
         fi
       fi
     done
+  done
 }
 
 # check for availability of sigaltstack to support our thread engine
@@ -233,6 +237,7 @@ simcc () {
    LDOPTS="$LDOPTS -mconsole"
    output="$output.exe"
    winbuild="yes"
+   CROSS=${CROSS:-"i586-mingw32msvc-"}
    SHARED_CFLAGS=''
  else
  case $uname in
@@ -360,7 +365,7 @@ EOF
  else
    # We are crosscompiling
    # add cross-compiler option(s)
-   prefixtools i586-mingw32msvc-
+   prefixtools $CROSS
    LDOPTS="$LDOPTS -mconsole"
    fibers=`check_fiber`
    output="rockboxui.exe"
@@ -3445,7 +3450,7 @@ else
    ""|"$CROSS_COMPILE")
      # simulator
    ;;
-   i586-mingw32msvc-)
+   ${CROSS})
      # cross-compile for win32
    ;;
    *)