gm2: Fix mc/mc.flex compilation on Solaris

gm2: Fix mc/mc.flex compilation on Solaris

The recent warning changes broke gm2 bootstrap on Solaris:

/vol/gcc/src/hg/master/local/gcc/m2/mc/mc.flex: In function 'handleFile':
/vol/gcc/src/hg/master/local/gcc/m2/mc/mc.flex:297:21: error: implicit
declaration of function 'alloca' [-Wimplicit-function-declaration]
  297 |   char *s = (char *)alloca (strlen (filename) + 2 + 1);
      |                     ^~~~~~

alloca needs <alloca.h> on Solaris, which isn't universally available.
Since mc.flex doesn't include any config header, I chose to switch to
__builtin_alloca instead.

/vol/gcc/src/hg/master/local/gcc/m2/mc/mc.flex:332:19: error: implicit
declaration of function 'index' [-Wimplicit-function-declaration]
  332 |   char   *p     = index(sdate, '\n');
      |                   ^~~~~

index is declared in <strings.h> on Solaris, again not a standard
header.  I simply switched to using strchr to avoid that issue.

Bootstrapped without regressions on i386-pc-solaris2.11,
sparc-sun-solaris2.11, x86_64-pc-linux-gnu, and
x86_64-apple-darwin23.1.0.

2023-12-03  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/m2:
	* mc/mc.flex [__GNUC__]: Define alloca as __builtin_alloca.
	(handleDate): Use strchr instead of index.