fix: quote template CMake variables

This fix addresses an issue where "cppstd" is undefined for pure C
recipes. Consequently, "set" sometimes leaves the
"conan_watched_std_variable" unset, leading to an error in the
subsequent string comparison:

```
CMake Error at .../conan_toolchain.cmake:64 (if):
  if given arguments:

    "READ_ACCESS" "STREQUAL" "MODIFIED_ACCESS" "AND" "NOT" "11" "STREQUAL"

  Unknown arguments specified
```

conan/tools/cmake/toolchain/blocks.py

@@ -274,11 +274,11 @@ class CppStdBlock(Block):
         # Define the C++ and C standards from 'compiler.cppstd' and 'compiler.cstd'
 
         function(conan_modify_std_watch variable access value current_list_file stack)
-            set(conan_watched_std_variable {{ cppstd }})
+            set(conan_watched_std_variable "{{ cppstd }}")
             if (${variable} STREQUAL "CMAKE_C_STANDARD")
-                set(conan_watched_std_variable {{ cstd }})
+                set(conan_watched_std_variable "{{ cstd }}")
             endif()
-            if (${access} STREQUAL "MODIFIED_ACCESS" AND NOT ${value} STREQUAL ${conan_watched_std_variable})
+            if ("${access}" STREQUAL "MODIFIED_ACCESS" AND NOT "${value}" STREQUAL "${conan_watched_std_variable}")
                 message(STATUS "Warning: Standard ${variable} value defined in conan_toolchain.cmake to ${conan_watched_std_variable} has been modified to ${value} by ${current_list_file}")
             endif()
             unset(conan_watched_std_variable)

test/functional/toolchains/cmake/test_cmake_toolchain.py

@@ -2112,3 +2112,42 @@ def build(self):
     c.run('build . --profile:host=android')
     assert 'VERSION ".0" format invalid.' not in c.out
     assert 'sdk: 1.0.0' in c.out
+
+
+@pytest.mark.tool("cmake")
+def test_cmake_toolchain_language_c():
+    client = TestClient()
+
+    conanfile = textwrap.dedent(r"""
+        from conan import ConanFile
+        from conan.tools.cmake import CMake, CMakeToolchain, cmake_layout
+
+        class Pkg(ConanFile):
+            settings = "os", "compiler", "arch", "build_type"
+            generators = "CMakeToolchain"
+            languages = "C"
+
+            def layout(self):
+                cmake_layout(self)
+
+            def build(self):
+                cmake = CMake(self)
+                cmake.configure()
+                cmake.build()
+        """)
+
+    cmakelists = textwrap.dedent("""
+    cmake_minimum_required(VERSION 3.15)
+    project(pkg C)
+    """)
+
+    client.save(
+        {
+            "conanfile.py": conanfile,
+            "CMakeLists.txt": cmakelists,
+        },
+        clean_first=True,
+    )
+
+    client.run("install . -s compiler.cstd=11")
+    client.run("build . -s compiler.cstd=11")