make FreeBSD's compiler clang

* there's not much documentation about what's in nim.cfg file
* the build.sh template was updated based on darwin

config/nim.cfg

@@ -100,7 +100,7 @@ hint[LineTooLong]=off
   @end
 @end
 
-@if macosx:
+@if macosx or freebsd:
   cc = clang
   tlsEmulation:on
   gcc.options.always = "-w -fasm-blocks"

tools/niminst/buildsh.tmpl

@@ -54,10 +54,16 @@ case $uos in
     myos="linux" 
     LINK_FLAGS="$LINK_FLAGS -ldl -lm"
     ;;
-  *freebsd* | *dragonfly* ) 
+  *dragonfly* )
     myos="freebsd"
     LINK_FLAGS="$LINK_FLAGS -lm"
     ;;
+  *freebsd* )
+    myos="freebsd"
+    CC="clang"
+    LINKER="clang"
+    LINK_FLAGS="$LINK_FLAGS -ldl -lm"
+    ;;
   *openbsd* )
     myos="openbsd" 
     LINK_FLAGS="$LINK_FLAGS -lm"