Install headers to a custom location to permit slotting with Qt4 version.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 45f8673..7832a3b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -94,7 +94,7 @@ set (LIB_SUFFIX "" CACHE STRING "Define suffix of library directory name (eg. '6
 set( LIB_INSTALL_DIR lib${LIB_SUFFIX} )
 set( PLUGIN_INSTALL_DIR lib${LIB_SUFFIX}/grantlee/${Grantlee_MAJOR_MINOR_VERSION_STRING} )
 set( BIN_INSTALL_DIR bin )
-set( INCLUDE_INSTALL_DIR include )
+set( INCLUDE_INSTALL_DIR include/grantlee5 )
 set( DATA_INSTALL_DIR share/apps )
 
 # set up RPATH/install_name_dir
diff --git a/templates/lib/CMakeLists.txt b/templates/lib/CMakeLists.txt
index 789205c..7045449 100644
--- a/templates/lib/CMakeLists.txt
+++ b/templates/lib/CMakeLists.txt
@@ -79,6 +79,7 @@ add_library(Grantlee5::Templates ALIAS Grantlee_Templates)
 generate_export_header(Grantlee_Templates)
 set_property(TARGET Grantlee_Templates PROPERTY EXPORT_NAME Templates)
 target_include_directories(Grantlee_Templates PRIVATE ../scriptabletags)
+target_include_directories(Grantlee_Templates INTERFACE "$<INSTALL_INTERFACE:${INCLUDE_INSTALL_DIR}>")
 
 if (BUILD_TESTS)
   set(GRANTLEE_TESTS_EXPORT "GRANTLEE_TEMPLATES_EXPORT")
diff --git a/textdocument/lib/CMakeLists.txt b/textdocument/lib/CMakeLists.txt
index 14163db..7c609f9 100644
--- a/textdocument/lib/CMakeLists.txt
+++ b/textdocument/lib/CMakeLists.txt
@@ -24,7 +24,7 @@ add_library(Grantlee_TextDocument SHARED
 generate_export_header(Grantlee_TextDocument)
 add_library(Grantlee::TextDocument ALIAS Grantlee_TextDocument)
 set_property(TARGET Grantlee_TextDocument PROPERTY EXPORT_NAME TextDocument)
-
+target_include_directories(Grantlee_TextDocument INTERFACE "$<INSTALL_INTERFACE:${INCLUDE_INSTALL_DIR}>")
 
 target_link_libraries(Grantlee_TextDocument
   LINK_PUBLIC Qt5::Gui