compilation resp. configure fails with "C++ compiler does not support C++14 standard....."

[MarioTrams]

Because I had troubles that mapnik-render (version 3.0.23) does not work for whatever reason I tried to build it from source as described here: https://github.com/mapnik/mapnik/wiki/UbuntuInstallation

During configure I get:
Checking if compiler (clang++-10) supports -std=c++14 flag... no
C++ compiler does not support C++14 standard (-std=c++14), which is required. Please upgrade your compiler

I have seen that this issue has been reported already multiple times.
For instance here https://ircama.github.io/osm-carto-tutorials/tile-server-ubuntu/ I found a note that one should set the environment variables like
export CXX="clang++-10" && export CC="clang-10"

I also installed clang-12, as this seems the latest available for Ubuntu 20.04. I set
export CXX="clang++-12" && export CC="clang-12"

I's not working either. Very likely the reason for the issue is not the reported one.
What can be wrong here?

Btw., not sure whether this relates to that: Initially configure did not work at all because it reported that python >= 3.5 is required.
Although there had been installed also newer versions of python, /usr/bin/python did point to /usr/bin/python2 on my system. I changed that link accordingly and I did not receive this error any more.

[ChrisSchmidt11]

When I ran into this error it turned out to be due to an error immediately above it, one of the dependencies needed to be updated. I can't remember which one, unfortunately, but I'd check the log file and see what happened right before this error came up.

[MarioTrams]

I did check config.log. The last messages are:

scons: Configure: Checking if compiler (clang++-12) supports -std=c++14 flag...
.sconf_temp/conftest_3f9f8e57ee3a30c900280a0be70bf928_0.cpp <-
|
|
|int main()
|{
|#if __cplusplus >= 201402L
| return 0;
|#else
| return -1;
|#endif
|}
|
|
clang++-12 -o .sconf_temp/conftest_3f9f8e57ee3a30c900280a0be70bf928_0.o -c -std=c++14 -DU_USING_ICU_NAMESPACE=0 -D_GLIBCXX_USE_CXX11_ABI=0 -DMAPNIK_MEMORY_MAPPED_FILE -DMAPNIK_HAS_DLCFN -Iinclude -Imason_packages/.link/include -Imason_packages/.link/include/freetype2 -Imason_packages/.link/include/libpng16 .sconf_temp/conftest_3f9f8e57ee3a30c900280a0be70bf928_0.cpp
clang++-12 -o .sconf_temp/conftest_3f9f8e57ee3a30c900280a0be70bf928_0_5855d121ec268ba4a14006bce293d2bb .sconf_temp/conftest_3f9f8e57ee3a30c900280a0be70bf928_0.o -Lsrc -Lsrc/json -Lsrc/wkt -Lmason_packages/.link/lib
/usr/bin/ld: cannot find -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)
scons: Configure: no
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

So lstdc++ cannot be found.

Because my initial problem with mapnik could be reduced to a wrong usage I don't need to compile it from the sources any more. However, this issue seems strange nonetheless....