Defining CMake variables from the command-line or from another CMake project,
such as `-DCMAKE_C_FLAGS= -DCMAKE_CXX_FLAGS=`, caused a link failure as ESP
CMake was unable to set its proper compilation flags.
Additional CMake compiler flags can now be provided by another project.
* Closes https://github.com/espressif/esp-idf/issues/7507
-nostdlib was removed in 9a2af7a and then accidentally added
in 1c2cc54.
The issue manifested itself as a failure to link some symbols from
libgcc, even though libgcc should implicitly appear at the end of the
linker command line when -nostdlib is not given.
