why I cannot install 'mnlogit' on my mac, thanks a lot.

[una]

> install.packages('mnlogit')
Package which is only available in source form, and may need compilation of C/C++/Fortran: ‘mnlogit’
Do you want to attempt to install these from sources? (Yes/no/cancel) yes
installing the source package ‘mnlogit’

trying URL 'https://cran.rstudio.com/src/contrib/mnlogit_1.2.6.tar.gz'
Content type 'application/x-gzip' length 506044 bytes (494 KB)
==================================================
downloaded 494 KB

* installing *source* package ‘mnlogit’ ...
** package ‘mnlogit’ successfully unpacked and MD5 sums checked
** using staged installation
** libs
clang++ -std=gnu++11 -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -I/usr/local/include -fopenmp -fPIC  -Wall -g -O2  -c hessian.cc -o hessian.o
clang: error: unsupported option '-fopenmp'
make: *** [hessian.o] Error 1
ERROR: compilation failed for package ‘mnlogit’
* removing ‘/Library/Frameworks/R.framework/Versions/3.6/Resources/library/mnlogit’
Warning in install.packages :
  installation of package ‘mnlogit’ had non-zero exit status

The downloaded source packages are in
	‘/private/var/folders/4r/jtyb5tbd78sbvv6p7t38h7jw0000gp/T/Rtmp3rvEI8/downloaded_packages’
Warning messages:
1: In doTryCatch(return(expr), name, parentenv, handler) :
  display list redraw incomplete
2: In doTryCatch(return(expr), name, parentenv, handler) :
  invalid graphics state
3: In doTryCatch(return(expr), name, parentenv, handler) :
  invalid graphics state
4: In doTryCatch(return(expr), name, parentenv, handler) :
  display list redraw incomplete
5: In doTryCatch(return(expr), name, parentenv, handler) :
  invalid graphics state
6: In doTryCatch(return(expr), name, parentenv, handler) :
  invalid graphics state