doc: Remove claim about ISO C

The patch to make -fcommon the default introduces a bogus claim into
the GCC documentation.

-fcommon was claimed to be incompatible with ISO C for preventing
duplicate definitions from being diagnosed.  It does, but as that
elicits undefined behaviour (the requirement that there shall be no
more than one external definition is not a constraint), ISO C does not
require any diagnostic for it.  In the absence of any other rule this
would violate, both -fcommon and -fno-common are fully compatible with
all versions of ISO C.

2019-11-21  Harald van Dijk  <harald@gigawatt.nl>

	* doc/invoke.texi (-fcommon): Remove claim about ISO C.

From-SVN: r278604

gcc/ChangeLog

+2019-11-21  Harald van Dijk  <harald@gigawatt.nl>
+
+	* doc/invoke.texi (-fcommon): Remove claim about ISO C.
+
 2019-11-21  Joseph Myers  <joseph@codesourcery.com>
 
 	* gimplify.c (expand_FALLTHROUGH_r, expand_FALLTHROUGH): Use

gcc/doc/invoke.texi

@@ -14141,9 +14141,9 @@ than one compilation unit.
 The @option{-fcommon} places uninitialized global variables in a common block.
 This allows the linker to resolve all tentative definitions of the same variable
 in different compilation units to the same object, or to a non-tentative
-definition.  This behavior does not conform to ISO C, is inconsistent with C++,
-and on many targets implies a speed and code size penalty on global variable
-references.  It is mainly useful to enable legacy code to link without errors.
+definition.  This behavior is inconsistent with C++, and on many targets implies
+a speed and code size penalty on global variable references.  It is mainly
+useful to enable legacy code to link without errors.
 
 @item -fno-ident
 @opindex fno-ident