xerces-c: fix handling of nsl library

[jcar87]

Summary

Changes to recipe: xerces-c all

Motivation

nsl is not a system library that is installed by default or found in all distros - so it shouldn't be listed as a system dependency without further safeguards.

Details

However, nsl appears to be completely unused even when found by the build system (there are no evaluations of the HAVE_LIBNSL macro, and no uses of any headers in the rpcsvc/* folder.
On systms where gcc is configured to pass --as-needed to the linker, the dependency is never encoded even if libnsl was found.

[jcar87]

On systms where gcc is configured to pass --as-needed to the linker, the dependency is never encoded even if libnsl was found.

We ensured this was the case for our images in conan-io/conan-docker-tools#584, however I suspect some of the Conan 1.x docker images are still legacy and not configured with this - otherwise the hook warning would not have been triggered.

I can confirmed that the Conan 2.x packages we published DO NOT encode a dependency on libnsl for the shared variant - so that's another indicator that it is not needed.

also the upstream .pc file does not mention this:
https://github.com/apache/xerces-c/blob/53c16411466bf90c62617831fe92ed0f41e70882/xerces-c.pc.in#L10

[conan-center-bot]

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All green in build 1 (6bc24fac186b0864b1f95ca8dcf92ac758b82cc8):

• xerces-c/3.2.5:
  All packages built successfully! (All logs)

• xerces-c/3.2.3:
  All packages built successfully! (All logs)

• xerces-c/3.2.4:
  All packages built successfully! (All logs)

• xerces-c/3.2.2:
  All packages built successfully! (All logs)

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All green in build 1 (6bc24fac186b0864b1f95ca8dcf92ac758b82cc8):

• xerces-c/3.2.5:
  All packages built successfully! (All logs)

• xerces-c/3.2.2:
  All packages built successfully! (All logs)

• xerces-c/3.2.4:
  All packages built successfully! (All logs)

• xerces-c/3.2.3:
  All packages built successfully! (All logs)

[github-actions]

Hooks produced the following warnings for commit 6bc24fa

xerces-c/3.2.5@#4534bf5ce931e79773601ba912a963a6

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c.so' links to system library 'm' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c-3.2.so' links to system library 'm' but it is not in cpp_info.system_libs.

xerces-c/3.2.3@#1f255728b15165538b7c364a117ff5a4

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c.so' links to system library 'm' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c-3.2.so' links to system library 'm' but it is not in cpp_info.system_libs.

xerces-c/3.2.4@#bf517826057577a146f6efe0f3ec43ff

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c.so' links to system library 'm' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c-3.2.so' links to system library 'm' but it is not in cpp_info.system_libs.

xerces-c/3.2.2@#e8a15e95f42afa20871a66ca880593bd

post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c.so' links to system library 'm' but it is not in cpp_info.system_libs.
post_package(): WARN: [MISSING SYSTEM LIBS (KB-H043)] Library './lib/libxerces-c-3.2.so' links to system library 'm' but it is not in cpp_info.system_libs.