環境
Mac OS 10.13.13 （High Sierra)

An error occurred while installing pg (1.0.0), and Bundler cannot continue.
Make sure that `gem install pg -v '1.0.0' --source 'https://rubygems.org/'` succeeds before bundling.

というエラーでこける。

エラーの全容

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/myname/.anyenv/envs/rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/pg-1.0.0/ext
/Users/myname/.anyenv/envs/rbenv/versions/2.3.1/bin/ruby -r ./siteconf20180905-43590-1wvyx66.rb extconf.rb
checking for pg_config... no
No pg_config... trying anyway. If building fails, please try again with
 --with-pg-config=/path/to/pg_config
checking for libpq-fe.h... no
Can't find the 'libpq-fe.h header
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/Users/myname/.anyenv/envs/rbenv/versions/2.3.1/bin/$(RUBY_BASE_NAME)
        --with-pg
        --without-pg
        --enable-windows-cross
        --disable-windows-cross
        --with-pg-config
        --without-pg-config
        --with-pg_config
        --without-pg_config
        --with-pg-dir
        --without-pg-dir
        --with-pg-include
        --without-pg-include=${pg-dir}/include
        --with-pg-lib
        --without-pg-lib=${pg-dir}/lib

To see why this extension failed to compile, please check the mkmf.log which can be found here:

/Users/myname/.anyenv/envs/rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-17/2.3.0-static/pg-1.0.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in
/Users/myname/.anyenv/envs/rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/pg-1.0.0 for inspection.
Results logged to
/Users/myname/.anyenv/envs/rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/extensions/x86_64-darwin-17/2.3.0-static/pg-1.0.0/gem_make.out

An error occurred while installing pg (1.0.0), and Bundler cannot continue.
Make sure that `gem install pg -v '1.0.0' --source 'https://rubygems.org/'` succeeds before bundling.

with native extention はOSにgem以外でインストールが必要ということなので、

brew install postgresql

か

brew reinstall postgresql

で解決。