use default c++ if specified (#48)

librocksdb_sys/build.sh

@@ -120,6 +120,7 @@ function compile_rocksdb() {
 }
 
 function find_stdcxx() {
+    if [[ "$CXX" = "" ]]; then
         if g++ --version &>/dev/null; then
             CXX=g++
         elif clang++ --version &>/dev/null; then
@@ -127,6 +128,7 @@ function find_stdcxx() {
         else
             error failed to find valid cxx compiler.
         fi
+    fi
 
     $CXX --print-file-name libstdc++.a
 }