Fix: Restore incremental builds with pkg-config ELPA (#7662)

* Fix: Restore incremental builds with pkg-config ELPA

* Change ${ELPA_PKG_VERSION} to ELPA_PKG_VERSION

Author: AsTonyshment

cmake/modules/FindELPA.cmake

@@ -7,6 +7,17 @@
 # Deprecated (TODO: Remove this part)
 # ========================================================================
 
+# Migrate caches created when FindPkgConfig used the public ELPA prefix.
+if(DEFINED CACHE{ELPA_LIBRARIES})
+  get_property(_elpa_libraries_cache_type CACHE ELPA_LIBRARIES PROPERTY TYPE)
+  if(_elpa_libraries_cache_type STREQUAL "INTERNAL")
+    unset(ELPA_LIBRARIES CACHE)
+    unset(ELPA_INCLUDE_DIRS CACHE)
+    unset(ELPA_LINK_LIBRARIES CACHE)
+  endif()
+endif()
+unset(_elpa_libraries_cache_type)
+
 # Compatible layer towards old manual routines
 if(DEFINED ELPA_DIR)
   message(WARNING "ELPA_DIR is deprecated and will be removed in the future release.")
@@ -47,20 +58,26 @@ endif()
 
 # ========================================================================
 
+# TODO: Make pkg-config discovery unconditional after removing the deprecated
+# manual discovery path.
 if(NOT ELPA_INCLUDE_DIRS)
   find_package(PkgConfig)
   if(NOT PKG_CONFIG_FOUND)
     message(FATAL_ERROR "Pkg-config is needed to get all information about the ELPA library")
   endif()
   # Find preferred library corresponding with ABACUS configuration first
   if(ENABLE_OPENMP)
-    pkg_search_module(ELPA REQUIRED IMPORTED_TARGET GLOBAL elpa_openmp elpa)
+    pkg_search_module(ELPA_PKG REQUIRED IMPORTED_TARGET GLOBAL elpa_openmp elpa)
   else()
-    pkg_search_module(ELPA REQUIRED IMPORTED_TARGET GLOBAL elpa)
+    pkg_search_module(ELPA_PKG REQUIRED IMPORTED_TARGET GLOBAL elpa)
   endif()
-  if(${ELPA_VERSION} VERSION_LESS "2021.05.001")
+  if(ELPA_PKG_VERSION VERSION_LESS "2021.05.001")
     message(FATAL_ERROR "ELPA version >= 2021.05.001 is required.")
   endif()
+  set(ELPA_INCLUDE_DIRS ${ELPA_PKG_INCLUDE_DIRS})
+  set(ELPA_LINK_LIBRARIES ${ELPA_PKG_LINK_LIBRARIES})
+  set(ELPA_LIBRARIES ${ELPA_PKG_LIBRARIES})
+  set(ELPA_VERSION ${ELPA_PKG_VERSION})
 endif()
 
 # Handle the QUIET and REQUIRED arguments and
@@ -73,18 +90,23 @@ if(ELPA_FOUND)
   list(GET ELPA_LINK_LIBRARIES 0 ELPA_LIBRARY)
   set(ELPA_INCLUDE_DIR ${ELPA_INCLUDE_DIRS})
   if(NOT TARGET ELPA::ELPA)
-    add_library(ELPA::ELPA UNKNOWN IMPORTED)
-    set_target_properties(ELPA::ELPA PROPERTIES
-      IMPORTED_LINK_INTERFACE_LANGUAGES "C"
-      IMPORTED_LOCATION "${ELPA_LIBRARY}"
-      INTERFACE_INCLUDE_DIRECTORIES "${ELPA_INCLUDE_DIR}")
+    # TODO: Remove the manual target fallback with the deprecated ELPA inputs.
+    if(TARGET PkgConfig::ELPA_PKG)
+      add_library(ELPA::ELPA ALIAS PkgConfig::ELPA_PKG)
+    else()
+      add_library(ELPA::ELPA UNKNOWN IMPORTED)
+      set_target_properties(ELPA::ELPA PROPERTIES
+        IMPORTED_LINK_INTERFACE_LANGUAGES "C"
+        IMPORTED_LOCATION "${ELPA_LIBRARY}"
+        INTERFACE_INCLUDE_DIRECTORIES "${ELPA_INCLUDE_DIR}")
+    endif()
   endif()
 endif()
 
 set(CMAKE_REQUIRED_INCLUDES ${CMAKE_REQUIRED_INCLUDES} ${ELPA_INCLUDE_DIR})
 
 # Compability workaround for ELPA_DIR
-# TODO: Remove this check
+# TODO: Remove this check with the deprecated ELPA_DIR path.
 include(CheckCXXSourceCompiles)
 check_cxx_source_compiles("
 #include <elpa/elpa_version.h>