Conanfile.py that sets a cmake flag?

If you want to create a recipe for a third party library and you want to modify any file from that library, including CMakeLists.txt, you can do that with tools.replace_in_file in your source method. The module tools is in the conans package.

I often use something like

tools.replace_in_file("sources/CMakeLists.txt", "project (ProjectName)",
                              '''project (ProjectName)
include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
conan_basic_setup()''')

such that I can use conan to get that third party library dependencies. You can add add_compile_options(-std=c++11) in the replacement string.

For instance, my full source method in a recipe for the vtk library is

def source(self):
        tools.get("https://www.vtk.org/files/release/{}/VTK-{}.tar.gz".format(
            ".".join(self.version.split(".")[:-1]), # Get only X.Y version, instead of X.Y.Z
            self.version))
        os.rename("VTK-{}".format(self.version), "sources")
        tools.replace_in_file("sources/CMakeLists.txt",
                              "project(VTK)",
                              """project(VTK)
include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
conan_basic_setup()
SET(CMAKE_INSTALL_RPATH "$ORIGIN")""")