Fix compilation on 32-bit systems

Author: TheAssassin

src/signing/CMakeLists.txt

@@ -11,3 +11,13 @@ target_link_libraries(signing
 target_include_directories(signing
     PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>/src
 )
+
+# won't work before project()! https://stackoverflow.com/a/39258832
+if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+    # GPGME requires this on 32-bit systems
+    # https://www.gnupg.org/documentation/manuals/gpgme/Largefile-Support-_0028LFS_0029.html
+    message(WARNING "Building on 32-bit system, adding special gpgme definitions")
+    target_compile_definitions(signing
+        PUBLIC -D_FILE_OFFSET_BITS=64 -DLARGEFILE_SOURCE
+    )
+endif()