Enable open source users to use jemalloc (github issue #438)

Summary: Currently open source rocksdb only builds with tcmalloc. This diff first checks if jemalloc is available. If it is, it compiles with jemalloc. If it isn't, it checks for tcmalloc.

Test Plan: Tried this out on my Ubuntu virtual machine and confirms that jemalloc is correctly detected and compiled.

Reviewers: MarkCallaghan, yhchiang, rven, sdong

Reviewed By: sdong

Subscribers: adamretter, meyering, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36789

build_tools/build_detect_platform

@@ -270,15 +270,20 @@ EOF
         JAVA_LDFLAGS="$JAVA_LDFLAGS -lnuma"
     fi
 
-    # Test whether tcmalloc is available
-    $CXX $CFLAGS -x c++ - -o /dev/null -ltcmalloc 2>/dev/null  <<EOF
-      int main() {}
-EOF
-    if [ "$?" = 0 ]; then
+    # Test whether jemalloc is available
+    if echo 'int main() {}' | $CXX $CFLAGS -x c++ - -o /dev/null -ljemalloc \
+      2>/dev/null; then 
+        PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -ljemalloc"
+        JAVA_LDFLAGS="$JAVA_LDFLAGS -ljemalloc"
+    else
+        # jemalloc is not available. Let's try tcmalloc
+        if echo 'int main() {}' | $CXX $CFLAGS -x c++ - -o /dev/null \
+          -ltcmalloc 2>/dev/null; then 
             PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -ltcmalloc"
             JAVA_LDFLAGS="$JAVA_LDFLAGS -ltcmalloc"
         fi
     fi
+fi
 
 # TODO(tec): Fix -Wshorten-64-to-32 errors on FreeBSD and enable the warning.
 # -Wshorten-64-to-32 breaks compilation on FreeBSD i386