Trying to build under Slackware64-14.2 with latest updates, but with cmake 3.5.2 it errors out.  Here is the cmake snippet:
----------------------------------------------------------------------------
-- Found UnixCommands: /usr/bin/bash  
-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29.2") 
CMake Error at CMakeLists.txt:201 (include):
  include could not find load file:

    GncPkgConfig


-- Checking for module 'glib-2.0>=2.40'
--   Found glib-2.0, version 2.46.2
-- Checking for module 'gio-2.0'
--   Found gio-2.0, version 2.46.2
-- Checking for module 'gobject-2.0>=2.20'
--   Found gobject-2.0, version 2.46.2
-- Checking for module 'gmodule-2.0>=2.20'
--   Found gmodule-2.0, version 2.46.2
-- Checking for module 'gthread-2.0>=2.20'
--   Found gthread-2.0, version 2.46.2
-- Checking for module 'libxml-2.0>=2.7.0'
--   Found libxml-2.0, version 2.9.5
-- Checking for module 'libxslt'
--   Found libxslt, version 1.1.29
-- Checking for module 'webkit2gtk-4.0'
--   Found webkit2gtk-4.0, version 2.20.5
CMake Error at CMakeLists.txt:241 (_pkg_create_imp_target):
  Unknown CMake command "_pkg_create_imp_target".


-- Configuring incomplete, errors occurred!
-----------------------------------------------------------------

Forgot to mention this is Gnucash 3.8.

The file is in common/cmake_modules. If it's missing then check your clone and make sure that you're on the maint branch and that you have no differences to origin.

common/cmake_modules/GncPkgConfig.cmake is missing from the 3.8 release tarball on sourceforge.  After copying it over from the git pull I was able to build with no problem.

Aha! I've retagged the release and re-spun the tarball because of https://github.com/Gnucash/gnucash/commit/67b508babecd12b8f16eda5abf8d5d5b16402c21. I'll fix the dist list and do it again.

Much thanks!