Support custom FMT_INC_DIR in pkgconfig and cmake configs (#1702)

* Support custom FMT_INC_DIR in pkgconfig and cmake configs

When CMAKE_INSTALL_INCLUDEDIR or FMT_INC_DIR override the header
installation directory, they should be used instead of
${CMAKE_INSTALL_PREFIX}/include in fmt-targets.cmake and fmt.pc.

* Put headers into FMT_INC_DIR/fmt rather than directly into FMT_INC_DIR

This makes FMT_INC_DIR an alias for CMAKE_INSTALL_INCLUDEDIR and
simplifies generation of pkgconfig and cmake configs.

* Do not split target_include_directories

CMakeLists.txt

@@ -44,6 +44,11 @@ if (MASTER_PROJECT AND NOT CMAKE_BUILD_TYPE)
               "CMAKE_C_FLAGS used) Debug Release RelWithDebInfo MinSizeRel.")
 endif ()
 
+include(GNUInstallDirs)
+set_verbose(FMT_INC_DIR ${CMAKE_INSTALL_INCLUDEDIR} CACHE STRING
+            "Installation directory for include files, a relative path "
+            "that will be joined to ${CMAKE_INSTALL_PREFIX}, or an arbitrary absolute path.")
+
 option(FMT_PEDANTIC "Enable extra warnings and expensive tests." OFF)
 option(FMT_WERROR "Halt the compilation with an error on compiler warnings."
        OFF)
@@ -198,7 +203,7 @@ target_compile_features(fmt INTERFACE ${FMT_REQUIRED_FEATURES})
 
 target_include_directories(fmt PUBLIC
   $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
-  $<INSTALL_INTERFACE:include>)
+  $<INSTALL_INTERFACE:${FMT_INC_DIR}>)
 
 set(FMT_DEBUG_POSTFIX d CACHE STRING "Debug library postfix.")
 
@@ -233,11 +238,10 @@ target_compile_features(fmt-header-only INTERFACE ${FMT_REQUIRED_FEATURES})
 
 target_include_directories(fmt-header-only INTERFACE
   $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
-  $<INSTALL_INTERFACE:include>)
+  $<INSTALL_INTERFACE:${FMT_INC_DIR}>)
 
 # Install targets.
 if (FMT_INSTALL)
-  include(GNUInstallDirs)
   include(CMakePackageConfigHelpers)
   set_verbose(FMT_CMAKE_DIR ${CMAKE_INSTALL_LIBDIR}/cmake/fmt CACHE STRING
               "Installation directory for cmake files, a relative path "
@@ -247,19 +251,10 @@ if (FMT_INSTALL)
   set(pkgconfig ${PROJECT_BINARY_DIR}/fmt.pc)
   set(targets_export_name fmt-targets)
 
-  set (INSTALL_TARGETS fmt)
-  if (TARGET fmt-header-only)
-    set(INSTALL_TARGETS ${INSTALL_TARGETS} fmt-header-only)
-  endif ()
-
   set_verbose(FMT_LIB_DIR ${CMAKE_INSTALL_LIBDIR} CACHE STRING
               "Installation directory for libraries, a relative path "
               "that will be joined to ${CMAKE_INSTALL_PREFIX}, or an arbitrary absolute path.")
 
-  set_verbose(FMT_INC_DIR ${CMAKE_INSTALL_INCLUDEDIR}/fmt CACHE STRING
-              "Installation directory for include files, a relative path "
-              "that will be joined to ${CMAKE_INSTALL_PREFIX}, or an arbitrary absolute path.")
-
   set_verbose(FMT_PKGCONFIG_DIR ${CMAKE_INSTALL_LIBDIR}/pkgconfig CACHE PATH
               "Installation directory for pkgconfig (.pc) files, a relative path "
               "that will be joined to ${CMAKE_INSTALL_PREFIX}, or an arbitrary absolute path.")
@@ -270,8 +265,8 @@ if (FMT_INSTALL)
     VERSION ${FMT_VERSION}
     COMPATIBILITY AnyNewerVersion)
 
-  join_paths(libdir_for_pc_file "\${exec_prefix}" "${CMAKE_INSTALL_LIBDIR}")
-  join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+  join_paths(libdir_for_pc_file "\${exec_prefix}" "${FMT_LIB_DIR}")
+  join_paths(includedir_for_pc_file "\${prefix}" "${FMT_INC_DIR}")
 
   configure_file(
     "${PROJECT_SOURCE_DIR}/support/cmake/fmt.pc.in"
@@ -281,6 +276,8 @@ if (FMT_INSTALL)
     ${PROJECT_SOURCE_DIR}/support/cmake/fmt-config.cmake.in
     ${project_config}
     INSTALL_DESTINATION ${FMT_CMAKE_DIR})
+
+  set(INSTALL_TARGETS fmt fmt-header-only)
   # Use a namespace because CMake provides better diagnostics for namespaced
   # imported targets.
   export(TARGETS ${INSTALL_TARGETS} NAMESPACE fmt::
@@ -301,7 +298,7 @@ if (FMT_INSTALL)
 
   install(FILES $<TARGET_PDB_FILE:${INSTALL_TARGETS}>
           DESTINATION ${FMT_LIB_DIR} OPTIONAL)
-  install(FILES ${FMT_HEADERS} DESTINATION ${FMT_INC_DIR})
+  install(FILES ${FMT_HEADERS} DESTINATION "${FMT_INC_DIR}/fmt")
   install(FILES "${pkgconfig}" DESTINATION "${FMT_PKGCONFIG_DIR}")
 endif ()