Follow-up from 4d2f8006777

Closes #6172

Fixed build by MCST lcc compiler on MCST Elbrus 2000 architecture and do
some code cleanup.

e2k (Elbrus 2000) - this is VLIW/EPIC architecture, like Intel Itanium
architecture.

Ref: https://en.wikipedia.org/wiki/Elbrus_2000

Closes https://github.com/curl/curl/pull/4576

Double-underscored or underscore plus uppercase letter at least.

... as they're claimed to be reserved.

Reported-by: patnyb on github

Fixes #4254
Closes #4255

system.h selects the proper Sun settings when __SUNPRO_C is defined. The
Sun compiler does not define it when compiling C++ files.  I'm adding a
check also on __SUNPRO_CC to allow curl to work properly also when used
in a C++ project on Sun Solaris.

Closes #3181

Bug: https://curl.haxx.se/mail/lib-2018-06/0100.html

Added a section to system.h guarded with __xlc__ for the IBM xml C
compiler. Before this change the section titled 'generic "safe guess" on
old 32 bit style' was used, which resulted in a wrong definition of
CURL_TYPEOF_CURL_SOCKLEN_T, and for 64-bit also CURL_TYPEOF_CURL_OFF_T
was wrong.

Compilation warnings fixed with this change:

  CC       libcurl_la-ftp.lo
"ftp.c", line 290.55: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 293.48: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 1070.49: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 1154.53: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"ftp.c", line 1187.51: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
  CC       libcurl_la-connect.lo
"connect.c", line 448.56: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"connect.c", line 516.66: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"connect.c", line 687.55: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
"connect.c", line 696.55: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.
  CC       libcurl_la-tftp.lo
"tftp.c", line 1115.33: 1506-280 (W) Function argument assignment between types "unsigned long* restrict" and "int*" is not allowed.

Closes #2637

With specific compiler options selecting the arch like -xarch=sparc on
newer compilers like Oracle Studio 12.4 there is no definition of
__sparcv8 but __sparcv8plus which means the V9 ISA, but limited to the
32ÎíÎñbit subset defined by the V8plus ISA specification, without the
Visual Instruction Set (VIS), and without other implementation-specific
ISA extensions. So it should be the same as __sparcv8.

Closes https://github.com/curl/curl/pull/2491

__SIZEOF_LONG__ was introduced in GCC 4.4, __LONG_MAX__ was introduced
in GCC 3.3.

Closes #2216

This reverts commit c97648b55080343bb371522bf4233e94a2a13a99.

SIZEOF_LONG should not be checked in system.h since that macro is only
defined when building libcurl.

Ref: https://github.com/curl/curl/pull/2186#issuecomment-354767080
Ref: https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html