Fix: InstallMode tests fail on some platforms

1) The ExternalProject_Add() command was called with
   UPDATE_COMMAND ";" which was not noticable on most platforms

2) On AIX/GCC, the executable did not link because symbols from
   imported libraries were assumed extern "C" (see commit 4fc47424)

Tests/InstallMode/Subproject.cmake

@@ -10,12 +10,7 @@ function(add_subproject _name)
 
   set(_maybe_NO_INSTALL)
   if(_arg_NO_INSTALL)
-    set(_maybe_NO_INSTALL "INSTALL_COMMAND")
-  else()
-    # This is a trick to get a valid call.
-    # Since we set UPDATE_COMMAND to ""
-    # explicitly below, this won't harm.
-    set(_maybe_NO_INSTALL "UPDATE_COMMAND")
+    set(_maybe_NO_INSTALL INSTALL_COMMAND "")
   endif()
 
   if(CMAKE_GENERATOR MATCHES "Ninja Multi-Config")
@@ -35,7 +30,9 @@ function(add_subproject _name)
   ExternalProject_Add("${_name}"
     DOWNLOAD_COMMAND      ""
     UPDATE_COMMAND        ""
-    ${_maybe_NO_INSTALL}  ""
+    UPDATE_DISCONNECTED   ON
+
+    "${_maybe_NO_INSTALL}"
 
     BUILD_ALWAYS          ON
 
@@ -67,7 +64,7 @@ function(add_subproject _name)
       # however, we need to explicitly inherit other parent
       # project's build settings.
       "-DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}"
-      ${_maybe_NINJA_MULTICONFIG_ARGS}
+      "${_maybe_NINJA_MULTICONFIG_ARGS}"
 
       # Subproject progress reports clutter up the output, disable
       "-DCMAKE_TARGET_MESSAGES:BOOL=OFF"

Tests/InstallMode/subpro_b_shared_lib/CMakeLists.txt

@@ -4,12 +4,18 @@ cmake_minimum_required(VERSION 3.20)
 project(shared_lib_project VERSION 2.3.4 LANGUAGES CXX)
 
 include(GNUInstallDirs)
+include(GenerateExportHeader)
 
 add_library(the_shared_lib SHARED
   "include/shared_lib.h"
   "src/shared_lib.cpp"
 )
 
+generate_export_header(the_shared_lib
+  BASE_NAME shared_lib
+  EXPORT_FILE_NAME include/shared_lib_export.h
+)
+
 set_target_properties(the_shared_lib
   PROPERTIES
     VERSION   "${PROJECT_VERSION}"
@@ -18,11 +24,14 @@ set_target_properties(the_shared_lib
 
 target_include_directories(the_shared_lib PUBLIC
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include>
   $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 
 install(
-  DIRECTORY   "${CMAKE_CURRENT_SOURCE_DIR}/include/"
+  DIRECTORY
+    "${CMAKE_CURRENT_SOURCE_DIR}/include/"
+    "${CMAKE_CURRENT_BINARY_DIR}/include/"
   DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
 )
 

Tests/InstallMode/subpro_b_shared_lib/include/shared_lib.h

@@ -1,3 +1,5 @@
 #pragma once
 
-void shared_hello();
+#include <shared_lib_export.h>
+
+void SHARED_LIB_EXPORT shared_hello();

Tests/InstallMode/subpro_c_nested_lib/subsubpro_c2_lib/CMakeLists.txt

@@ -18,6 +18,9 @@ target_link_libraries(the_c2_lib
     the_c1_lib
 )
 
+# This is to fix an issue on AIX/GCC (see commit 4fc47424)
+set_property(TARGET the_c2_lib PROPERTY NO_SYSTEM_FROM_IMPORTED 1)
+
 target_include_directories(the_c2_lib PUBLIC
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
   $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>

Tests/InstallMode/subpro_d_executable/CMakeLists.txt

@@ -17,6 +17,9 @@ target_link_libraries(the_executable PRIVATE
   the_c2_lib
 )
 
+# This is to fix an issue on AIX/GCC (see commit 4fc47424)
+set_property(TARGET the_executable PROPERTY NO_SYSTEM_FROM_IMPORTED 1)
+
 install(
   TARGETS
     the_executable