Python extensions use CC and LDSHARED from the sysconfig module to
build. When Spack installs Python, it replaces the Spack compiler
wrappers in these values with the underlying compilers (since these
wrappers are not useful outside of the context of running Spack).
In order to use the Spack compiler wrappers when building Python
extensions with Spack, Spack sets the LDSHARED environment variable
when running `Python.setup_py` (which overrides sysconfig). However,
many Python extensions use an alternative method to build (namely
PythonPackage.setup_py), which meant that LDSHARED was not set (and
RPATHs were not inserted for dependencies).
This commit makes the following changes:
* Sets LDSHARED in the environment: this applies to all commands
executed during the build, rather than for a single command
invocation
* Updates the logic to set LDSHARED: this replaces the compiler
executable in LDSHARED with the Spack compiler wrapper. This
means that for some externally-built instances of Python,
Spack will now switch to using the Spack wrappers when building
extensions. The behavior is expected to be the same for Spack-
built instances of Python.
* Performs similar modifications for LDCXXSHARED (to ensure RPATHs
are included for C++ codes)
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