cmake: export targets

The build system creates two exported targets:
- The executable FluidSynth::fluidsynth
- The library FluidSynth::libfluidsynth

A downstream project using CMake can find and link the library target
directly with cmake (without needing pkg-config) this way:

~~~
project(sample LANGUAGES C)
find_package ( FluidSynth )
if (FluidSynth_FOUND)
  add_executable( sample sample.c )
  target_link_libraries ( sample PRIVATE FluidSynth::libfluidsynth )
endif ()
~~~

After installing fluidsynth in a prefix like "$HOME/Fluidsynth3":
cmake -DCNAKE_PREFIX_PATH="$HOME/Fluidsynth3/;..."

Instead installing, the build directory can be used directly, for
instance:

cmake -DFluidSynth_DIR="$HOME/fluidsynth-2.2.2/build/" ...

CMakeLists.txt

@@ -856,6 +856,29 @@ configure_file ( fluidsynth.pc.in
 install ( FILES ${CMAKE_BINARY_DIR}/fluidsynth.pc
 	DESTINATION ${LIB_INSTALL_DIR}/pkgconfig )
 
+# Exported targets for cmake: find_package(FluidSynth)
+# when installed, use CMAKE_PREFIX_PATH=fluidsynth-prefix;...
+# to use the build directory directly set the FluidSynth_DIR variable instead.
+
+# targets in the build directory
+export(EXPORT FluidSynthTargets
+   FILE "${CMAKE_CURRENT_BINARY_DIR}/FluidSynthTargets.cmake"
+   NAMESPACE FluidSynth::
+)
+
+include(CMakePackageConfigHelpers)
+write_basic_package_version_file(
+    FluidSynthConfigVersion.cmake
+    VERSION ${LIB_VERSION_INFO}
+    COMPATIBILITY SameMajorVersion
+)
+
+configure_file(FluidSynthConfig.cmake.in FluidSynthConfig.cmake @ONLY)
+install(FILES "${CMAKE_CURRENT_BINARY_DIR}/FluidSynthConfig.cmake"
+              "${CMAKE_CURRENT_BINARY_DIR}/FluidSynthConfigVersion.cmake"
+    DESTINATION ${LIB_INSTALL_DIR}/cmake/fluidsynth
+)
+
 # Extra targets for Unix build environments
 if ( UNIX )
     if ( DEFINED FLUID_DAEMON_ENV_FILE)

FluidSynthConfig.cmake.in

@@ -0,0 +1,11 @@
+# for the find_dependency() macro:
+# include(CMakeFindDependencyMacro)
+# it has the same syntax as find_package:
+# find_dependency(MYDEP REQUIRED)
+
+# define variables for configuration options:
+# set(network-enabled @enable-network@)
+
+# finally, include the targets/version files
+include("${CMAKE_CURRENT_LIST_DIR}/FluidSynthTargets.cmake")
+include("${CMAKE_CURRENT_LIST_DIR}/FluidSynthConfigVersion.cmake")

src/CMakeLists.txt

@@ -399,12 +399,14 @@ target_link_libraries ( fluidsynth
 
 if ( MACOSX_FRAMEWORK )
   install ( TARGETS fluidsynth libfluidsynth
+    EXPORT FluidSynthTargets
     RUNTIME DESTINATION ${BIN_INSTALL_DIR}
     FRAMEWORK DESTINATION ${FRAMEWORK_INSTALL_DIR}
     ARCHIVE DESTINATION ${FRAMEWORK_INSTALL_DIR}
 )
 else ( MACOSX_FRAMEWORK )
   install ( TARGETS fluidsynth libfluidsynth
+    EXPORT FluidSynthTargets
     RUNTIME DESTINATION ${BIN_INSTALL_DIR}
     LIBRARY DESTINATION ${LIB_INSTALL_DIR}
     ARCHIVE DESTINATION ${LIB_INSTALL_DIR}
@@ -413,6 +415,22 @@ else ( MACOSX_FRAMEWORK )
    install ( FILES ${public_main_HEADER} DESTINATION ${INCLUDE_INSTALL_DIR} )
 endif ( MACOSX_FRAMEWORK )
 
+# Exported targets.
+
+# build_interface: for the libfluidsynth target when imported from the build directory.
+# install_interface: for the target when imported from the installed directory.
+target_include_directories(libfluidsynth PUBLIC
+    "$<BUILD_INTERFACE:${CMAKE_SOURCE_DIR}/include/;${CMAKE_SOURCE_DIR}/include/fluidsynth/;${CMAKE_BINARY_DIR}/include/;${CMAKE_BINARY_DIR}/include/fluidsynth/>"
+    "$<INSTALL_INTERFACE:${INCLUDE_INSTALL_DIR}>"
+)
+
+# installation of the exported targets
+install(EXPORT FluidSynthTargets
+        FILE FluidSynthTargets.cmake
+        NAMESPACE FluidSynth::
+        DESTINATION ${LIB_INSTALL_DIR}/cmake/fluidsynth
+)
+
 # ******* Auto Generated Lookup Tables ******
 
 include(ExternalProject)