Condition cmake dependency

conanfile.py

@@ -1,17 +1,19 @@
 from conan import ConanFile
+import shutil
 
 class ApngasmRecipe(ConanFile):
     settings = 'os', 'compiler', 'build_type', 'arch'
 
     def requirements(self):
         self.requires("zlib/1.2.13")
         self.requires("libpng/1.6.40")
-        self.requires("boost/1.73.0")
+        self.requires("boost/1.69.0")
         self.generators = ['CMakeToolchain', 'CMakeDeps']
 
     def build_requirements(self):
-        # https://stackoverflow.com/questions/42123509/cmake-finds-boost-but-the-imported-targets-not-available-for-boost-version
-        self.tool_requires("cmake/[>=3.27]")
+        self.build_requires("b2/4.5.0")
+        if not shutil.which('cmake'):
+            self.tool_requires("cmake/[>=3.27]")
 
     def build(self):
         build_type = 'Release'