xerces-c: hotfix - honor shared=False option

[SpaceIm]

Fix for #13502 (comment)

It comes from https://github.com/apache/xerces-c/blob/v3.2.3/cmake/XercesDLL.cmake#L23 and how BUILD_SHARED_LIBS is injected in toolchain file (see conan-io/conan#11937 & conan-io/conan#11840).

---

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[conan-center-bot]

All green in build 2 (4cb744066d1656ccacf87e5ed064adfa423318bc):

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

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

[github-actions]

Hooks produced the following warnings for commit 4cb7440

xerces-c/3.2.3

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

xerces-c/3.2.2

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