Build gRPC shared against abseil static: undefined reference to nan@@GLIBC_2.2.5 in `charconv.cc` (`absl_strings`) on Linux

**Describe the bug**

Undefined reference to `nan` in `charconv.cc` (`absl_strings`) while building `gRPC` shared against `abseil` static on Linux with gcc-10:

```
[ 98%] Linking CXX shared library ../lib/libgrpc++.so
/usr/bin/ld: /home/conan/w/BuildSingleReference/.conan/data/abseil/20211102.0/_/_/package/b173bbda18164d49a449ffadc1c9e817f49e819d/lib/libabsl_strings.a(charconv.cc.o): undefined reference to symbol 'nan@@GLIBC_2.2.5'
/usr/bin/ld: /lib/x86_64-linux-gnu/libm.so.6: error adding symbols: DSO missing from command line
```

**Steps to reproduce the bug**

- Linux and C++ lib without nan implementation in `std::` namespace I guess.
- Build abseil static
- Build gRPC shared

**What version of Abseil are you using?**

20211102.0

**What operating system and version are you using**

Some linux distribution, ubuntu 20 I guess (a docker of a CI I don't control so sorry not much information).

**What compiler and version are you using?**

gcc-10

**What build system are you using?**

CMake

**Additional context**

I think that `absl_strings` may depend on libm instead of libstdc++ only, due to https://github.com/abseil/abseil-cpp/blob/fbbb5865a562c9a9167d71c1cf56b82025a8f065/absl/strings/charconv.cc#L86-L91

So I guess it's sufficient to link directly to libm in case of, and add this in CMakeLists of absl_strings, so that libm is properly propagated in CMake imported target if static:

```cmake
find_library(LIBM m)

absl_cc_library(
NAME
strings
...
LINKOPTS
$<$:-lm>
...
```

The issue is in absl/strings/charconv.cc, which uses `nan` from libm. The CMakeLists.txt for the absl_strings library needs to link to libm when built statically. Start by examining absl/strings/CMakeLists.txt and the absl_cc_library macro. Understand how LINKOPTS are propagated. The fix likely involves adding `-lm` to the link options for the strings target. Test by building abseil statically and then building gRPC shared against it to verify the undefined reference is resolved.

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