Improve memkind library detection (#9134)

Summary:
Improve memkind library detection in build_detect_platform:

- The current position of -lmemkind does not work with all versions of gcc
- LDFLAGS allows specifying non-standard library path through EXTRA_LDFLAGS

After the change, the options match TBB detection.
This is a follow-up to https://github.com/facebook/rocksdb/issues/6214.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9134

Reviewed By: ajkr, mrambacher

Differential Revision: D32192028

fbshipit-source-id: 115fafe8d93f1fe6aaf80afb32b2cb67aad074c7

build_tools/build_detect_platform

@@ -469,7 +469,7 @@ EOF
 
     if ! test $ROCKSDB_DISABLE_MEMKIND; then
         # Test whether memkind library is installed
-        $CXX $PLATFORM_CXXFLAGS $COMMON_FLAGS -lmemkind -x c++ - -o test.o 2>/dev/null  <<EOF
+        $CXX $PLATFORM_CXXFLAGS $LDFLAGS -x c++ - -o test.o -lmemkind 2>/dev/null  <<EOF
           #include <memkind.h>
           int main() {
             memkind_malloc(MEMKIND_DAX_KMEM, 1024);