From 39c6275341ac22f99342b92d0096c1bc5d753460 Mon Sep 17 00:00:00 2001
From: Ryan Burns <rtburns@protonmail.com>
Date: Sun, 14 Jun 2026 16:54:22 -0700
Subject: [PATCH] Respect relative/absolute CMAKE_INSTALL_INCLUDEDIR

CMAKE_INSTALL_INCLUDEDIR is already used in some places, but not
consistently, causing headers to be installed to multiple locations when
it is specified differently from the default "include" value.

Additionally, upstream GNUInstallDirs documentations notes that install
dirs may be specified as relative (to install prefix) or absolute paths,
so we should only append to the install prefix when non-absolute paths
are given.
---
 cmake/libappimage.pc.in               |  5 ++---
 cmake/scripts.cmake                   |  2 +-
 src/CMakeLists.txt                    | 11 +++++++++++
 src/libappimage/CMakeLists.txt        |  4 ++--
 src/libappimage_shared/CMakeLists.txt |  2 +-
 5 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/cmake/libappimage.pc.in b/cmake/libappimage.pc.in
index 32682d86..6bff1e1a 100644
--- a/cmake/libappimage.pc.in
+++ b/cmake/libappimage.pc.in
@@ -1,7 +1,6 @@
 prefix=@CMAKE_INSTALL_PREFIX@
-# Use prefix as base path to make the package relocatable
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=@CMAKE_INSTALL_REL_LIBDIR@
+includedir=@CMAKE_INSTALL_REL_INCLUDEDIR@
 
 Name: @PROJECT_NAME@
 Description: AppImage management and desktop integration
diff --git a/cmake/scripts.cmake b/cmake/scripts.cmake
index 0228e739..6482440a 100644
--- a/cmake/scripts.cmake
+++ b/cmake/scripts.cmake
@@ -257,6 +257,6 @@ function(configure_libappimage_module target)
     target_include_directories(${target}
         PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include/>
         PRIVATE $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/src/libappimage>
-        INTERFACE $<INSTALL_INTERFACE:include/>
+        INTERFACE $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/>
     )
 endfunction()
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 9737205b..17a6cb3f 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -47,6 +47,17 @@ install(
 
 if(NOT LIBAPPIMAGE_SHARED_ONLY)
     # pkg-config
+    # Use prefix as base path to make the package relocatable, when possible
+    if(IS_ABSOLUTE "${CMAKE_INSTALL_INCLUDEDIR}")
+        set(CMAKE_INSTALL_REL_INCLUDEDIR "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
+    else()
+        set(CMAKE_INSTALL_REL_INCLUDEDIR "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+    endif()
+    if(IS_ABSOLUTE "${CMAKE_INSTALL_LIBDIR}")
+        set(CMAKE_INSTALL_REL_LIBDIR "${CMAKE_INSTALL_FULL_LIBDIR}")
+    else()
+        set(CMAKE_INSTALL_REL_LIBDIR "\${prefix}/${CMAKE_INSTALL_LIBDIR}")
+    endif()
     configure_file("${PROJECT_SOURCE_DIR}/cmake/libappimage.pc.in" "${PROJECT_BINARY_DIR}/libappimage.pc" @ONLY)
 
     install(FILES "${PROJECT_BINARY_DIR}/libappimage.pc"
diff --git a/src/libappimage/CMakeLists.txt b/src/libappimage/CMakeLists.txt
index 47005221..992b5fad 100644
--- a/src/libappimage/CMakeLists.txt
+++ b/src/libappimage/CMakeLists.txt
@@ -70,13 +70,13 @@ install(
 # install public headers
 install(
     DIRECTORY ${PROJECT_SOURCE_DIR}/include/appimage/
-    DESTINATION include/appimage
+    DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/appimage"
     COMPONENT libappimage-dev
 )
 
 install(
     DIRECTORY ${PROJECT_BINARY_DIR}/generated-headers/appimage/
-    DESTINATION include/appimage
+    DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/appimage"
     COMPONENT libappimage-dev
 )
 
diff --git a/src/libappimage_shared/CMakeLists.txt b/src/libappimage_shared/CMakeLists.txt
index ef7238bb..234414d3 100644
--- a/src/libappimage_shared/CMakeLists.txt
+++ b/src/libappimage_shared/CMakeLists.txt
@@ -13,7 +13,7 @@ add_library(libappimage_shared STATIC
 set_target_properties(libappimage_shared PROPERTIES PREFIX "")
 target_include_directories(libappimage_shared PUBLIC
     $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
-    $<INSTALL_INTERFACE:include>
+    $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
 )
 set_property(TARGET libappimage_shared PROPERTY PUBLIC_HEADER ${libappimage_shared_public_header})
 target_link_libraries(libappimage_shared PRIVATE libappimage_hashlib)