configure: handle spaces correctly in $source_path

Add very few missing quotes where IFS/globs were applied to arbitrary
user input (commands, arguments, but not assignments or case $var in.
In fn_makelink $dn/$2/$f don't have IFS/globs).

While configure now handles it correctly, there are 2 issues:
- eval opt=\"$opt\"  coalesces IFS chars (--prefix="$HOME/x   y").
- Even with the eval removed, (gnu) make still fails to find it.

I made few quick experiments with quoting these values in config.mak,
but that had no impact, and make -d was not helpful. Was worth a try.

So no spaces in paths, but at least configure can handle the src path.

configure

@@ -438,7 +438,7 @@ fi
 
 if test -z "$build_cross"; then
   CONFTEST=./conftest$EXESUF
-  if ! $cc -o $CONFTEST $source_path/conftest.c ; then
+  if ! $cc -o $CONFTEST "$source_path/conftest.c" ; then
     echo "configure: error: '$cc' failed to compile conftest.c."
   else
     cc_name="$($CONFTEST compiler)"
@@ -553,7 +553,7 @@ fcho "Switches            " "$tcc_switches"
 fcho "Config              " "${confvars# }"
 echo "Creating config.mak and config.h"
 
-version=`head $source_path/VERSION`
+version=$(head "$source_path/VERSION")
 
 cat >config.mak <<EOF
 # Automatically generated by configure - do not modify
@@ -726,13 +726,13 @@ fn_makelink()
 	 ;;
     esac
 
-    ln -sfn $tgt $2 || ( echo "ln failed. Using cp instead."; cp -f $1/$2 $2 )
+    ln -sfn "$tgt" $2 || ( echo "ln failed. Using cp instead."; cp -f "$1/$2" $2 )
 }
 
 if test "$source_path_used" = "yes" ; then
   FILES="Makefile lib/Makefile tests/Makefile tests/tests2/Makefile tests/pp/Makefile"
   for f in $FILES ; do
-    fn_makelink $source_path $f
+    fn_makelink "$source_path" $f
   done
 fi