Compile warning: `ar` default

[AltJ]

On Ubuntu 18.04

...
make[3]: Entering directory '/eccoin/src/univalue'
  CXX      lib/libunivalue_la-univalue.lo                  
  CXX      lib/libunivalue_la-univalue_get.lo
  CXX      lib/libunivalue_la-univalue_read.lo     
  CXX      lib/libunivalue_la-univalue_write.lo                                        
  CXXLD    libunivalue.la       
ar: `u' modifier ignored since `D' is the default (see `U')
make[3]: Leaving directory '/eccoin/src/univalue'
...

...
make[3]: Entering directory '/eccoin/src/rsm'
  CXX      lib/librsm_la-recursive_shared_mutex.lo
  CXXLD    librsm.la
ar: `u' modifier ignored since `D' is the default (see `U')
make[3]: Leaving directory '/eccoin/src/rsm'
make[3]: Entering directory '/eccoin/src/secp256k1'
gcc -I. -g -O2 -Wall -Wextra -Wno-unused-function -c src/gen_context.c -o gen_context.o
gcc gen_context.o -o gen_context
./gen_context
  CC       src/libsecp256k1_la-secp256k1.lo
  CCLD     libsecp256k1.la
/usr/bin/ar: `u' modifier ignored since `D' is the default (see `U')
make[3]: Leaving directory '/eccoin/src/secp256k1'
...