rc: also use clang with homebrew/linuxbrew

ref:
https://github.com/Homebrew/linuxbrew/issues/415#issuecomment-106058234

rc/bashrc

@@ -809,6 +809,7 @@ erase-mbr() {
 # place for this.
 if hash clang 2>/dev/null; then
     export CC=clang
+    export HOMEBREW_CC=clang
 fi
 
 if hash clang++ 2>/dev/null; then

rc/zshrc

@@ -798,6 +798,7 @@ erase-mbr() {
 # place for this.
 if hash clang 2>/dev/null; then
     export CC=clang
+    export HOMEBREW_CC=clang
 fi
 
 if hash clang++ 2>/dev/null; then