Unfortunately, the headers are not only installed into a different folder, the CMake build system is broken. If this gets unnoticed, it is probably because utf8.h gets installed into the standard include directory, so it is found anyway, even if the dependency to utfcpp is not correctly specified. To see the issue, we need to use a non-standard include path, e.g. using a custom prefix. This is not a theoretical problem, currently, TagLib cannot be built without hacks on macOS using Homebrew because of this.

It can be reproduced like this, first it is shown that it works correctly for version 3.2.5

git clone git@github.com:nemtrif/utfcpp.git
cd utfcpp
git checkout v3.2.5
cmake -B build -DCMAKE_INSTALL_PREFIX=$HOME/custom-prefix -DUTF8_TESTS=OFF
make -C build
make -C build install

mkdir ../utfcpp-client
cd ../utfcpp-client
cat >CMakeLists.txt <<EOF
cmake_minimum_required(VERSION 3.5.0)
project(utfcpp-client)
find_package(utf8cpp)
add_executable(client client.cpp)
target_link_libraries(client utf8::cpp)
EOF
cat >client.cpp <<EOF
#include <string>
#include <iostream>
#include <utf8.h>
int main(int argc, char *argv[]) {
  std::u16string utf16string = {0x41, 0x0448, 0x65e5, 0xd834, 0xdd1e};
  std::string u = utf8::utf16to8(utf16string);
  std::cout << u << std::endl;
  return 0;
}
EOF
cmake -B build -DCMAKE_PREFIX_PATH=$HOME/custom-prefix
make -C build
# Run build/client, it works

Now the same with 4.0.1

cd ../utfcpp
rm -rf $HOME/custom-prefix build
git checkout v4.0.1
cmake -B build -DCMAKE_INSTALL_PREFIX=$HOME/custom-prefix -DUTF8_TESTS=OFF
make -C build
make -C build install

cd ../utfcpp-client
cmake -B build -DCMAKE_PREFIX_PATH=$HOME/custom-prefix

-- Configuring done
CMake Error at CMakeLists.txt:5 (target_link_libraries):
Target "client" links to:
utf8::cpp
but the target was not found. Possible reasons include:
* There is a typo in the target name.
* A find_package call is missing for an IMPORTED target.
* An ALIAS target is missing.

Looking into $HOME/custom-prefix/share/utf8cpp/cmake/utf8cppTargets.cmake one sees that the imported target is now utf8cpp::utf8cpp and no longer utf8::cpp (as set previously in utf8cppConfig.cmake). So let's change this.

sed -i 's/utf8::cpp/utf8cpp::utf8cpp/' CMakeLists.txt

cmake -B build -DCMAKE_PREFIX_PATH=$HOME/custom-prefix

-- Configuring done
CMake Error in CMakeLists.txt:
Imported target "utf8cpp::utf8cpp" includes non-existent path
"/home/urs/custom-prefix/include/utf8cpp"
in its INTERFACE_INCLUDE_DIRECTORIES. Possible reasons include:

The path was deleted, renamed, or moved to another location.

An install or uninstall procedure did not complete successfully.

The installation package was faulty and references files it does not
provide.

Another possibility would be to use utf8cpp as the name of the imported CMake target. But this will only work with v3.2.5, with v4.0.1 we get

(..)/utfcpp-client/client.cpp:3:10: fatal error: utf8.h: No such file or directory
3 | #include <utf8.h>

I would suggest the following changes to fix it:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6fdd906..2ca50a6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,7 +10,7 @@ include(GNUInstallDirs)
 
 target_include_directories(utf8cpp INTERFACE
     "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/source>"
-    $<INSTALL_INTERFACE:include/utf8cpp>
+    $<INSTALL_INTERFACE:include>
 )
 
 include(CMakePackageConfigHelpers)
diff --git a/utf8cppConfig.cmake.in b/utf8cppConfig.cmake.in
index 9c15f36..4bdb9c4 100644
--- a/utf8cppConfig.cmake.in
+++ b/utf8cppConfig.cmake.in
@@ -2,3 +2,7 @@
 
 include("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@Targets.cmake")
 check_required_components("@PROJECT_NAME@")
+
+if(NOT TARGET utf8::cpp)
+    add_library(utf8::cpp ALIAS utf8cpp::utf8cpp)
+endif()

After 4.0.0 headers gets installed into include/utf8 using CMake #112

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