Revert topic 'CMakePackageConfigHelpers_build_tree'

It will be replaced by an INSTALL_PREFIX option topic instead.
Merge topic 'CMakePackageConfigHelpers_build_tree' into next

3bfa6de9 Revert topic 'CMakePackageConfigHelpers_build_tree'
CMakePackageConfigHelpers: Remove unused variable
CMakePackageConfigHelpers: restructure documentation and document commands
CMakePackageConfigHelpers: Add INSTALL_PREFIX option

This option instructs configure_package_config_file to consider paths
that are not absolute as relative to the INSTALL_PREFIX directory
instead of relative to the CMAKE_INSTALL_PREFIX directory.

If this argument is not passed, the default behaviour is to use
the CMAKE_INSTALL_PREFIX variable.

The new signature is therefore:

  configure_package_config_file(<input> <output> INSTALL_DESTINATION <path>
                                                 [PATH_VARS <var1> <var2> ... <varN>]
                                                 [NO_SET_AND_CHECK_MACRO]
                                                 [NO_CHECK_REQUIRED_COMPONENTS_MACRO]
                                                 [INSTALL_PREFIX <path>])
CMakePackageConfigHelpers: Add unit tests for INSTALL_PREFIX option
Merge topic 'CMakePackageConfigHelpers-INSTALL_PREFIX' into next

65d23e58 CMakePackageConfigHelpers: Add unit tests for INSTALL_PREFIX option
609ba044 CMakePackageConfigHelpers: Add INSTALL_PREFIX option
477babe0 CMakePackageConfigHelpers: restructure documentation and document commands
64eca30d CMakePackageConfigHelpers: Remove unused variable
liblzma: Fix compilation with PGI compiler

- sha265.c is using some C99 specific features, in particular static
array dimensions in a function parameter array (see section 6.7.5-7
of the C99 spec).  A #ifndef check was in place to prevent compilation
under MSVC but it actually needed to check for C99 compliance instead.
Even still, the C99 code fails on a few compilers (PGI being one) so for
compatibility reasons, the C99 version of the function declaration is
removed entirely, leaving only the C89 version.

- CHECK_SYMBOL_EXISTS is used to determine the presense of bswap
functions from byteswap.h.  Most compilers re-dedefine the bswap_N
functions as a __bswap_N function implemented by the compiler.  Since
bswap_N is usually defined as a macro then it's mere presence passes
the check.  Some versions of the PGI compiler though have shipped
broken headers for byteswap.h, in particular 11.3 for x64 linux
provides byteswap.h but is missing an associated bits/byteswap.h which
causes some of the bswap_N macros to be defined but broken and unusable.
The bswap_N checks have been converted to CHECK_SOURCE_COMPILES to
ensure that the bswap_N calls are actually usable and not just merely
defined.
Merge topic 'liblzma-pgi-compiler' into next

2dd17a54 liblzma: Fix compilation with PGI compiler