Can't install nokogiri gem on mac (libxml2) - ruby

Question

I'm trying to install the nokogiri gem, but it says:
libxml2 is missing. Please locate mkmf.log to investigate how it is failing.
When I try to install libxml2 with brew, it says:
Warning: libxml2 2.9.5 is already installed
Earlier, it said that it was installing a keg-only version because the macOS already had a version of libxml2.
So, gem can't find any copies of libxml2, but brew finds 2?
How do I tell gem where to find libxml2?
looking at mkmf.log, I see that it seems to not know about iconv?
have_iconv?: checking for iconv using --with-opt-* flags... -------------------
- yes
"gcc -o conftest -I/Users/brianp/.rvm/rubies/ruby-2.1.10/include/ruby-2.1.0/x86_64-darwin16.0 -I/Users/brianp/.rvm/rubies/ruby-2.1.10/include/ruby-2.1.0/ruby/backward -I/Users/brianp/.rvm/rubies/ruby-2.1.10/include/ruby-2.1.0 -I. -I/usr/local/opt/libyaml/include -I/usr/local/opt/readline/include -I/usr/local/opt/libksba/include -I/usr/local/opt/openssl/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT -O3 -fno-fast-math -ggdb3 -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wunused-variable -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wshorten-64-to-32 -Wimplicit-function-declaration -Wdivision-by-zero -Wextra-tokens -fno-common -pipe -O3 -Wall -Wcast-qual -Wwrite-strings -Wconversion -Wmissing-noreturn -Winline conftest.c -L. -L/Users/brianp/.rvm/rubies/ruby-2.1.10/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/readline/lib -L/usr/local/opt/libksba/lib -L/usr/local/opt/openssl/lib -L. -fstack-protector -L/usr/local/opt/libyaml/lib -L/usr/local/opt/readline/lib -L/usr/local/opt/libksba/lib -L/usr/local/opt/openssl/lib -lruby.2.1.0 -lpthread -ldl -lobjc "
Undefined symbols for architecture x86_64:
"_iconv", referenced from:
_main in conftest-311f4b.o
"_iconv_open", referenced from:
_main in conftest-311f4b.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: #include <stdlib.h>
4: #include <iconv.h>
5:
6: int main(void)
7: {
8: iconv_t cd = iconv_open("", "");
9: iconv(cd, NULL, NULL, NULL, NULL);
10: return EXIT_SUCCESS;
11: }
/* end */
again, it says I have iconv, but then it crashes?
I must be misunderstanding a message somewhere.