getting error on gem install

[Mitch VanDuyn]

MacOS 10.11.6

make || true 2>&1
/Library/Developer/CommandLineTools/usr/bin/make  all-recursive
Making all in src
/bin/sh ../libtool --tag=CXX   --mode=compile g++ -DHAVE_CONFIG_H -I. -I..   -D_THREAD_SAFE -I/usr/local/include  -pthread -ffast-math -O3 -MT pHash.lo -MD -MP -MF .deps/pHash.Tpo -c -o pHash.lo pHash.cpp
libtool: compile:  g++ -DHAVE_CONFIG_H -I. -I.. -D_THREAD_SAFE -I/usr/local/include -pthread -ffast-math -O3 -MT pHash.lo -MD -MP -MF .deps/pHash.Tpo -c pHash.cpp  -fno-common -DPIC -o pHash.o
In file included from pHash.cpp:25:
In file included from ./pHash.h:50:
../CImg.h:2160:44: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
      static char min() { return (char)(-1L<<(8*sizeof(char)-1)); }
                                        ~~~^
../CImg.h:2161:46: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
      static char max() { return ~((char)(-1L<<(8*sizeof(char)-1))); }
                                          ~~~^