cmake: Properly set pkg-config requires when configured with ICU

In this case the pkg-config file needs to set icu-cu in the `Required`
field, and needs to add the flag `-DCXXOPTS_USE_UNICODE` to the `Cflags`
field.

cmake/cxxopts.cmake

@@ -154,6 +154,10 @@ function(cxxopts_install_logic)
     set(CPACK_DEBIAN_COMPRESSION_TYPE "xz")
 
     set(PKG_CONFIG_FILE_NAME "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc")
+    if(CXXOPTS_USE_UNICODE_HELP)
+        set(PKG_CONFIG_REQUIRES "icu-cu")
+        set(PKG_CONFIG_EXTRA_CFLAGS "-DCXXOPTS_USE_UNICODE")
+    endif()
     configure_file("${PackagingTemplatesDir}/pkgconfig.pc.in" "${PKG_CONFIG_FILE_NAME}" @ONLY)
     install(FILES "${PKG_CONFIG_FILE_NAME}"
             DESTINATION "${CMAKE_INSTALL_LIBDIR_ARCHIND}/pkgconfig"

packaging/pkgconfig.pc.in

@@ -5,4 +5,5 @@ Name: @PROJECT_NAME@
 Description: @PROJECT_DESCRIPTION@
 URL: @PROJECT_HOMEPAGE_URL@
 Version: @PROJECT_VERSION@
-Cflags: -I${includedir}
+Requires: @PKG_CONFIG_REQUIRES@
+Cflags: -I${includedir} @PKG_CONFIG_EXTRA_CFLAGS@