Update mozconfig.

.mozconfig

@@ -20,9 +20,19 @@ export WIN_UCRT_REDIST_DIR="/c/Program Files (x86)/Windows Kits/10/Redist/ucrt/D
 elif test `uname -s` = Linux; then
 #x86 Specific Options
 if test `uname -m` = x86_64; then
+
+if test -d "$HOME/.mozbuild/clang/bin"; then
 export CC=$HOME/.mozbuild/clang/bin/clang
 export CXX=$HOME/.mozbuild/clang/bin/clang++
+else
+export CC=clang
+export CXX=clang++
+fi
+
+if test -f "$HOME/.mozbuild/nasm/nasm"; then
 export NASM=$HOME/.mozbuild/nasm/nasm
+fi
+
 ac_add_options --enable-optimize="-O2 -march=nocona -mtune=nocona -w"
 ac_add_options --target=x86_64-pc-linux-gnu
 #ppc64le specific options