Cross-building needs that include and library paths to be computed 
at this package's build time and not at runtime usage of this cmake support.

Code inspired from cmake files of KDE Framework 5 various subparts.

--- lang/cpp/src/GpgmeppConfig.cmake.in.in.ORIG
+++ lang/cpp/src/GpgmeppConfig.cmake.in.in
@@ -58,19 +58,41 @@
 unset(_targetsNotDefined)
 unset(_expectedTargets)
 
+
+# Compute the installation prefix relative to this file.
+get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_FILE}" PATH)
+# Use original install prefix when loaded through a
+# cross-prefix symbolic link such as /lib -> /usr/lib.
+get_filename_component(_realCurr "${_IMPORT_PREFIX}" REALPATH)
+get_filename_component(_realOrig "/usr/lib/cmake/Gpgmepp" REALPATH)
+if(_realCurr STREQUAL _realOrig)
+  set(_IMPORT_PREFIX "/usr/lib/cmake/Gpgmepp")
+endif()
+unset(_realOrig)
+unset(_realCurr)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+if(_IMPORT_PREFIX STREQUAL "/")
+  set(_IMPORT_PREFIX "")
+endif()
+
 # Create imported target Gpgmepp
 add_library(Gpgmepp SHARED IMPORTED)
 
 set_target_properties(Gpgmepp PROPERTIES
-  INTERFACE_INCLUDE_DIRECTORIES "@resolved_includedir@/gpgme++;@resolved_includedir@"
-  INTERFACE_LINK_LIBRARIES "pthread;@resolved_libdir@/libgpgme.so;@LIBASSUAN_LIBS@"
-  IMPORTED_LOCATION "@resolved_libdir@/libgpgmepp@libsuffix@"
+  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/gpgme++"
+  INTERFACE_LINK_LIBRARIES "pthread;${_IMPORT_PREFIX}/lib/libgpgme@libsuffix@;@LIBASSUAN_LIBS@"
+  IMPORTED_LOCATION "${_IMPORT_PREFIX}/lib/libgpgmepp@libsuffix@"
 )
 
 if(CMAKE_VERSION VERSION_LESS 2.8.12)
   message(FATAL_ERROR "This file relies on consumers using CMake 2.8.12 or greater.")
 endif()
 
+# Cleanup temporary variables.
+set(_IMPORT_PREFIX)
+
 # Loop over all imported files and verify that they actually exist
 foreach(target ${_IMPORT_CHECK_TARGETS} )
   foreach(file ${_IMPORT_CHECK_FILES_FOR_${target}} )
--- lang/qt/src/QGpgmeConfig.cmake.in.in.ORIG
+++ lang/qt/src/QGpgmeConfig.cmake.in.in
@@ -58,19 +58,41 @@
 unset(_targetsNotDefined)
 unset(_expectedTargets)
 
+
+# Compute the installation prefix relative to this file.
+get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_FILE}" PATH)
+# Use original install prefix when loaded through a
+# cross-prefix symbolic link such as /lib -> /usr/lib.
+get_filename_component(_realCurr "${_IMPORT_PREFIX}" REALPATH)
+get_filename_component(_realOrig "/usr/lib/cmake/QGpgme" REALPATH)
+if(_realCurr STREQUAL _realOrig)
+  set(_IMPORT_PREFIX "/usr/lib/cmake/QGpgme")
+endif()
+unset(_realOrig)
+unset(_realCurr)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+get_filename_component(_IMPORT_PREFIX "${_IMPORT_PREFIX}" PATH)
+if(_IMPORT_PREFIX STREQUAL "/")
+  set(_IMPORT_PREFIX "")
+endif()
+
 # Create imported target QGpgme
 add_library(QGpgme SHARED IMPORTED)
 
 set_target_properties(QGpgme PROPERTIES
-  INTERFACE_INCLUDE_DIRECTORIES "@resolved_includedir@/qgpgme;@resolved_includedir@"
+  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/qgpgme"
   INTERFACE_LINK_LIBRARIES "Gpgmepp;Qt5::Core"
-  IMPORTED_LOCATION "@resolved_libdir@/libqgpgme@libsuffix@"
+  IMPORTED_LOCATION "${_IMPORT_PREFIX}/libqgpgme@libsuffix@"
 )
 
 if(CMAKE_VERSION VERSION_LESS 2.8.12)
   message(FATAL_ERROR "This file relies on consumers using CMake 2.8.12 or greater.")
 endif()
 
+# Cleanup temporary variables.
+set(_IMPORT_PREFIX)
+
 # Loop over all imported files and verify that they actually exist
 foreach(target ${_IMPORT_CHECK_TARGETS} )
   foreach(file ${_IMPORT_CHECK_FILES_FOR_${target}} )