Fix: Installed Symlink LIB (ECP-WarpX#2626)

The latest patch to these routines broke our library alias in installs.

By default, this variable is relative and needs the prefix appended. In some cases, e.g., if externally set, it can already be absolute. In that case, we skip adding the prefix.

CMakeLists.txt

@@ -343,9 +343,14 @@ if(WarpX_LIB)
     else()
         set(mod_ext "so")
     endif()
+    if(IS_ABSOLUTE CMAKE_INSTALL_LIBDIR)
+        set(ABS_INSTALL_LIB_DIR ${CMAKE_INSTALL_LIBDIR})
+    else()
+        set(ABS_INSTALL_LIB_DIR ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR})
+    endif()
     install(CODE "file(CREATE_LINK
         $<TARGET_FILE_NAME:shared>
-        ${CMAKE_INSTALL_LIBDIR}/libwarpx.${lib_suffix}.${mod_ext}
+        ${ABS_INSTALL_LIB_DIR}/libwarpx.${lib_suffix}.${mod_ext}
         COPY_ON_ERROR SYMBOLIC)")
 endif()