CheckIncludeFiles: Use modern C declaration for 'main' (#15058)

Otherwise some compilers will incorrectly fail the checks when certain
warning/error flags are enabled.
Help: Remove stray content from 'add_custom_command' docs

Remove leftover generator expression documentation that is now in
the cmake-generator-expressions(7) manual.
Help: Revise and format 'add_custom_command' docs

Format the reStructuredText markup manually.  Organize the command
options into a definition list.  Use inline markup to cross-reference
related documents.
Help: Document add_custom_command PRE_BUILD/PRE_LINK for exe/lib only (#15059)

State explicitly that the PRE_LINK mode is not for targets created by
the add_custom_target command.  The existing wording for PRE_BUILD being
treated as PRE_LINK by non-VS generators will now imply this restriction
for PRE_BUILD too.
Fix used value warning.

packageFileName no longer has only one value in this function.  Move the
declaration into the loop and remove its initial and unused asignment.
ExternalData: Drop 'not security software' disclaimer (#15060)

Let clients do their own research on the current strength of each
hash algorithm to choose what is best for their needs.

Suggested-by: Nico Schlömer <nico.schloemer@gmail.com>
file: Refactor internal implementation of file(STRINGS)

Make room for encoding support.
file: Add ENCODING option to file(STRINGS) command (#10519)

Support extraction of UTF-8 strings.
Merge topic 'file-strings-encoding'

5b30ec28 file: Add ENCODING option to file(STRINGS) command (#10519)
ffa373e7 file: Refactor internal implementation of file(STRINGS)
Merge topic 'CheckIncludeFiles-main-prototype'

bf89f910 CheckIncludeFiles: Use modern C declaration for 'main' (#15058)
Merge topic 'doc-add_custom_command'

5bf1c5cc Help: Document add_custom_command PRE_BUILD/PRE_LINK for exe/lib only (#15059)
2a58c872 Help: Revise and format 'add_custom_command' docs
cb88742d Help: Remove stray content from 'add_custom_command' docs
Merge topic 'fix_value_not_used'

2cac483e Fix used value warning.
Merge topic 'ExternalData-drop-security-disclaimer'

f2ccc14b ExternalData: Drop 'not security software' disclaimer (#15060)