gcc could not find header files on mac os catalina

I am installing a package (called CLASS, widely used in cosmology) which cannot be compiled with Apple's gcc.

I tried installing gcc by homebrew (gcc-9) and anaconda separately. But both of them could not find the standard C-library files such as stdio.h, math.h etc. I saw this problem very common in mac. I found the standard C-library files at /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/. Focusing on anaconda gcc (version 4.8.5), I then copied the files to /Users/satadru/anaconda2/include/. Now simple C-codes, such as a 'hello world' code runs fine. But when I tried to install the CLASS package, I get error arising from the problematic library files: stdio.h, math.h!! I get same error when I try to install the package using homebrew gcc-9 (after copying the library files to its concerned directory). I know that many people could run the package on catalina without any issue.

Now I have the following questions:

1. Are all standard C-library header files same? In other words, be it anaconda gcc 4.8.5, or gcc-9, the header files are same? Do the header files vary on different os, say linux or mac os?

2. Does anaconda gcc look for the header files at /Users/satadru/anaconda2/include/?

3. Before copying all the header files to /Users/satadru/anaconda2/include/, the directory had a few present there. Why does not anaconda gcc installation place all of its own header files in this directory at the time of installation? Is it same when we do it on linux os?

4. How to solve my problem? I contacted the owner of the package and he says he himself runs the code on mac os catalina, but never faced this issue.

More information:

xcode-select --install

gives

xcode-select: error: command line tools are already installed, use "Software Update" to install updates

which gcc gives /Users/satadru/anaconda2/bin/gcc

gcc -v gives

Using built-in specs.COLLECT_GCC=gccCOLLECT_LTO_WRAPPER=/Users/satadru/anaconda2/bin/../libexec/gcc/x86_64-apple-darwin11.4.2/4.8.5/lto-wrapperTarget: x86_64-apple-darwin11.4.2Configured with: ./configure --prefix=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac --with-gxx-include-dir=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/gcc/include/c++ --bindir=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/bin --datarootdir=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/share --libdir=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/lib --with-gmp=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac --with-mpfr=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac --with-mpc=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac --with-isl=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac --with-cloog=/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac --with-boot-ldflags='-Wl,-headerpad_max_install_names -Wl,-L/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/lib -Wl,-L/usr/lib' --with-stage1-ldflags='-Wl,-headerpad_max_install_names -Wl,-L/Users/ray/mc-x64-3.5/conda-bld/gcc-4.8_1477649012852/_b_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/lib -Wl,-L/usr/lib' --enable-checking=release --with-tune=generic --disable-multilibThread model: posixgcc version 4.8.5 (GCC)