Fix handling of tools.build:defines for (Ninja) multi-config CMake

The CMake generator-expressions that results from the values given to
Conan's `tools.build:defines` configuration will now properly be put
into quotes to form a CMake string. (Note: CMake's generator-expressions
should never contain any (non-escaped) quotes.)

conan/tools/cmake/toolchain/blocks.py

@@ -724,7 +724,7 @@ class ExtraFlagsBlock(Block):
         {% if defines %}
         {% if config %}
         {% for define in defines %}
-        add_compile_definitions($<$<CONFIG:{{config}}>:"{{ define }}">)
+        add_compile_definitions("$<$<CONFIG:{{config}}>:{{ define }}>")
         {% endfor %}
         {% else %}
         add_compile_definitions({% for define in defines %} "{{ define }}"{% endfor %})