cmake: more small tidy-ups and fixes

[vszakats]

• tidy up two MATCHES expression by avoiding macros expansion and
  adding quotes. Then convert then to STREQUAL to match other places
  in the code doing the same checks.

• fix setting _ALL_SOURCE for AIX to match what autotools does.

• delete stray _ALL_SOURCE reference from lib/config_riscos.h

• simplify/fix two STREQUAL "" checks.
  The one in the openssl_check_symbol_exists() macro succeeded
  regardless of the value. The other could return TRUE when
  CMAKE_OSX_SYSROOT was undefined.

• delete code for CMake versions (<3.7) we no longer support.

• prefer LIST(APPEND ...) to extend CURL_LIBS.

• use CURL_LIBS to add the network lib for Haiku.
  Before this patch it was done via raw C flags. I could not test this.

• move _WIN32_WINNT-related code next to each other.
  It also moves detection to the top, allowing more code to use
  the result.

• merge two WIN32 blocks.

• rename internal variables to underscore + lowercase.

• unwrap a line, indent, whitespace.

Closes #14450

[vszakats]

This odd AIX construct is also generated by autoreconf into lib/curl_config.h.in:

/* Define to 1 if OS is AIX. */
#ifndef _ALL_SOURCE
#  undef _ALL_SOURCE
#endif

What may be the purpose of this code?

Besides, autotools (like CMake after this PR) does define _ALL_SOURCE for AIX, which should work regardless of this code. What am I missing?

[dfandrich]

The AIX 7.1 headers do things like defining _XOPEN_SOURCE to 700, _POSIX_C_SOURCE to 200809L, setting _LARGE_FILE_API and a few more defines when it sees _ALL_SOURCE. So, it looks similar to what glibc does for _DEFAULT_SOURCE.

[vszakats]

Thanks Dan.

Do you know what purpose the #ifndef / #undef combo may have?
After staring at it for a while I cannot figure it out. It seems, just defining _ALL_SOURCE should be enough for AIX, or is it not?

[vszakats]

Ah OK, I think I get it, autotools converts the odd construct to this, when defined:

/* Define to 1 if OS is AIX. */
#ifndef _ALL_SOURCE
#  define _ALL_SOURCE 1
#endif

I still cannot test it, but the CMake method of passing -D_ALL_SOURCE via the command-line would likely work without such trick. (Unless, AIX also defines it to a different value without checking if it's already set).

Probably making it -D_ALL_SOURCE=1 would be useful to sync it more with autotools.

[dfandrich]

All the AIX references to _ALL_SOURCE are either #ifdef or defined() so it doesn't need to be set to a value like 1. AIX seems to set _ALL_SOURCE itself if none of _XOPEN_SOURCE, _POSIX_SOURCE or ANSI_C_SOURCE is already set.

You should get yourself a Compile Farm account to help with this kind of testing. They have two different AIX versions available as well as other obscure machines.

[vszakats]

Thanks, they answer the questions and I've closed #14461 as not necessary.

Also for the AIX tip!