CMake: fix manpage installation from parent projects

.github/workflows/main.yml

@@ -186,6 +186,22 @@ jobs:
         run: |
           make amalgamated_timing
 
+  CMake-subproject:
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        shared: [ 'Off', 'On' ]
+    steps:
+      - uses: actions/checkout@v4
+      - name: build and install from a parent project
+        run: |
+          cmake -S demo/cmake_subproject -B build-subproject \
+            -DBUILD_SHARED_LIBS=${{ matrix.shared }} -DCMAKE_BUILD_TYPE=Release \
+            -DCMAKE_INSTALL_PREFIX="$PWD/install-subproject" \
+            -DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON
+          cmake --build build-subproject --parallel 2
+          cmake --install build-subproject
+
   CMake:
     runs-on: ${{ matrix.os }}
     strategy:

CMakeLists.txt

@@ -191,7 +191,7 @@ write_basic_package_version_file(
 # Windows uses a different help sytem.
 if((NOT WIN32) AND (NOT CMAKE_HOST_WIN32))
 # install manpage (not gzipped, some BSD's do not want it compressed?)
-install(FILES ${CMAKE_SOURCE_DIR}/doc/tommath.3
+install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/doc/tommath.3
         DESTINATION ${CMAKE_INSTALL_MANDIR}/man3/
 )
 endif()

demo/cmake_subproject/CMakeLists.txt

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: Unlicense
+
+cmake_minimum_required(VERSION 3.10)
+project(libtommath_parent C)
+
+add_subdirectory(../.. libtommath)