Complete full implementation of pre-building/pre-installing stages of…

… packages (#63)

This commit should complete what is "Use Case 3: Configure/build pointing to a
subset of already installed TriBITS packages in same repo."

To complete this, this commit geneates wrapper <Package>Config.cmake files for
TriBITS-compatible external packages.  This provides the info needed for
downstream internal TriBITS package's <Package>Config.camke files to get the
upstream TriBITS-compatible external package.

This commit makes the earlier test:

  TriBITS_TribitsExampleProject_External_Package_by_Package

fully pass.

However, we need to still add more tests, documentation and error checking but
the initial functionality should be complete.

tribits/core/package_arch/TribitsProcessEnabledTpls.cmake

@@ -156,6 +156,9 @@ macro(tribits_process_enabled_tribits_compatible_tpl  TPL_NAME)
   set(TPL_${TPL_NAME}_PARTS_ALREADY_SET FALSE)  # ToDo: Take out?
   if (NOT TPL_${TPL_NAME}_PARTS_ALREADY_SET)
     find_package(${TPL_NAME} CONFIG REQUIRED)
+    tribits_write_tribits_compatible_external_package_config_file(${TPL_NAME})
+    tribits_generate_tpl_version_file_and_add_package_config_install_targets(
+      ${TPL_NAME})
     set(TPL_${TPL_NAME}_PARTS_ALREADY_SET TRUE)
   endif()
 endmacro()
@@ -253,3 +256,28 @@ function(tribits_generate_tpl_version_file_and_add_package_config_install_target
   tribits_extpkg_install_config_version_file(${TPL_NAME}
     "${tplConfigVersionFile}")
 endfunction()
+
+
+# Generate a <Package>Config.cmake wrapper file for a fully TriBITS-compliant
+# external package and put it in the external_packages/ directory so it can be
+# included like any external package/TPL.
+#
+function(tribits_write_tribits_compatible_external_package_config_file  tplName)
+  # Create <tplName>Config.cmake file
+  set(configFileStr "")
+  string(APPEND configFileStr
+    "include(CMakeFindDependencyMacro)\n" )
+  if (${externalPkg}_DIR)
+    string(APPEND configFileStr
+      "set(${tplName}_DIR \"${${tplName}_DIR}\")\n" )
+  endif()
+  string(APPEND configFileStr
+    "find_dependency(${tplName} CONFIG REQUIRED)\n"
+    )
+  set(buildDirExternalPkgsDir
+    "${${PROJECT_NAME}_BINARY_DIR}/${${PROJECT_NAME}_BUILD_DIR_EXTERNAL_PKGS_DIR}")
+  set(tplConfigFile
+    "${buildDirExternalPkgsDir}/${tplName}/${tplName}Config.cmake")
+  file(WRITE "${tplConfigFile}" "${configFileStr}")
+
+endfunction()