$ gem install pg
Fetching: pg-0.21.0.gem (100%)
Building native extensions.  This could take a while...
ERROR:  Error installing pg:
        ERROR: Failed to build gem native extension.

    current directory: /home/vagrant/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/pg-0.21.0/ext
/home/vagrant/.rbenv/versions/2.4.1/bin/ruby -r ./siteconf20170906-6891-137sln2.rb extconf.rb
checking for pg_config... yes
Using config values from /usr/bin/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=/home/vagrant/.rbenv/versions/2.4.1/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:

  /home/vagrant/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/extensions/x86_64-linux/2.4.0-static/pg-0.21.0/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /home/vagrant/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/pg-0.21.0 for inspection.
Results logged to /home/vagrant/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/extensions/x86_64-linux/2.4.0-static/pg-0.21.0/gem_make.out

$ sudo yum install postgresql-devel
Loaded plugins: fastestmirror
base                                                                                                               | 3.6 kB  00:00:00     
extras                                                                                                             | 3.4 kB  00:00:00     
updates                                                                                                            | 3.4 kB  00:00:00     
Loading mirror speeds from cached hostfile
 * base: www.ftp.ne.jp
 * extras: www.ftp.ne.jp
 * updates: www.ftp.ne.jp
Resolving Dependencies
--> Running transaction check
---> Package postgresql-devel.x86_64 0:9.2.18-1.el7 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

==========================================================================================================================================
 Package                                Arch                         Version                             Repository                  Size
==========================================================================================================================================
Installing:
 postgresql-devel                       x86_64                       9.2.18-1.el7                        base                       950 k

Transaction Summary
==========================================================================================================================================
Install  1 Package

Total download size: 950 k
Installed size: 3.9 M
Is this ok [y/d/N]: y
Downloading packages:
postgresql-devel-9.2.18-1.el7.x86_64.rpm                                                                           | 950 kB  00:00:00     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : postgresql-devel-9.2.18-1.el7.x86_64                                                                                   1/1 
  Verifying  : postgresql-devel-9.2.18-1.el7.x86_64                                                                                   1/1 

Installed:
  postgresql-devel.x86_64 0:9.2.18-1.el7                                                                                                  

Complete!

[vagrant@localhost ruby]$ gem install pg
Building native extensions.  This could take a while...
Successfully installed pg-0.21.0
Parsing documentation for pg-0.21.0
Installing ri documentation for pg-0.21.0
Done installing documentation for pg after 0 seconds
1 gem installed