Build system tweaks; fix use of brk(2)/sbrk(2) feature test

There is a feature test for brk(2)/sbrk(2), but it was not checked
for in one place in vmbest.c, causing libdll to fail to build on
FreeBSD aarch64 because the features/dll output{...}end block
failed to link. This commit allows libdll to build on that system,
though another mysterious build failure apparently remains.
https://github.com/ksh93/ksh/issues/154

src/lib/libast/include/vmalloc.h,
src/lib/libast/vmalloc/vmbest.c:
- Add missing '#if _mem_sbrk' directives to disable uses of sbrk(2)
  on systems that have removed this deprecated interface.

src/cmd/builtin/features/pty,
src/lib/libast/features/common,
src/lib/libast/features/float,
src/lib/libast/features/lib,
src/lib/libast/features/sfio,
src/lib/libast/features/sizeof:
- Add a fail clause to more 'tst - output{' blocks so they write an
  informative #error directive if they fail to compile and write
  required header identifiers. This should avoid much more obscure
  compile errors later on. (re: e20c0c6b)

.gitignore:
- Add pattern for emacs #backup# files.

.gitignore

@@ -57,3 +57,4 @@ bin/execrate
 *.project
 *.core
 core
+**/#*#

src/cmd/builtin/features/pty

@@ -54,6 +54,8 @@ tst - output{
 			}
 		return 0;
 	}
+}end fail{
+	echo '#error The output block in src/cmd/builtin/features/pty failed to compile'
 }end
 
 extern	_getpty		char*		(int*, int, mode_t, int)

src/lib/libast/features/common

@@ -352,6 +352,8 @@ tst	- output{
 		printf("\n");
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #2 in src/lib/libast/features/common failed to compile'
 }end
 
 tst	- -DN=1 - -DN=0 output{

src/lib/libast/features/float

@@ -894,6 +894,8 @@ tst	- note{ missing floating point limits }end output{
 
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #1 in src/lib/libast/features/float failed to compile'
 }end
 
 tst	- note{ double exponent bitfoolery }end output{
@@ -928,6 +930,8 @@ tst	- note{ double exponent bitfoolery }end output{
 			}
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #2 in src/lib/libast/features/float failed to compile'
 }end
 
 tst	- note{ long double exponent bitfoolery }end output{
@@ -962,6 +966,8 @@ tst	- note{ long double exponent bitfoolery }end output{
 			}
 		return 0;
 	}
+}end fail{
+	echo '#error Output block #3 in src/lib/libast/features/float failed to compile'
 }end
 
 tst	- -DN=1 - -DN=2 note{ _ast_fltmax_t maximum integral type }end output{

src/lib/libast/features/lib

@@ -699,6 +699,8 @@ tst	- output{
 		return 0;
 	}
 
+}end fail{
+	echo '#error The output block in src/lib/libast/features/lib failed to compile'
 }end
 
 tst	no64 -D_LARGEFILE64_SOURCE note{ largefile 64 broken }end execute{

src/lib/libast/features/sfio

@@ -14,6 +14,8 @@ tst	- note{ number of bits in pointer }end output{
 		printf("#define _ptr_bits	%d\n", sizeof(char*) * 8);
 		return 0;
 	}
+}end fail{
+	echo '#error The output block in src/lib/libast/features/sfio failed to compile'
 }end
 
 tst	tmp_rmfail note{ open files cannot be removed }end execute{

src/lib/libast/features/sizeof

@@ -10,4 +10,6 @@ tst	- note{ sizeof(integral-type) }end output{
 		printf("#define _ast_sizeof_intmax_t	%d\n", sizeof(_ast_intmax_t));
 		return 0;
 	}
+}end fail{
+	echo '#error The output block in src/lib/libast/features/sizeof failed to compile'
 }end

src/lib/libast/include/vmalloc.h

@@ -136,7 +136,9 @@ extern Vmethod_t*	Vmprofile;	/* profiling memory usage	*/
 
 extern Vmdisc_t*	Vmdcsystem;	/* get memory from the OS	*/
 extern Vmdisc_t*	Vmdcheap;	/* get memory from Vmheap	*/
+#if _mem_sbrk
 extern Vmdisc_t*	Vmdcsbrk;	/* like Vmdcsystem - legacy use	*/
+#endif
 
 extern Vmalloc_t	_Vmheap;	/* heap region - use with care! */
 extern Vmalloc_t*	Vmheap;		/* = &_Vmheap - safe to use	*/

src/lib/libast/vmalloc/vmbest.c

@@ -1265,10 +1265,13 @@ static Void_t* mmapmem(Void_t* caddr, size_t csize, size_t nsize, Mmdisc_t* mmdc
 		}
 	}
 	else if(nsize == 0)
-	{	Vmuchar_t	*addr = (Vmuchar_t*)sbrk(0);
+	{
+#if _mem_sbrk
+		Vmuchar_t	*addr = (Vmuchar_t*)sbrk(0);
 		if(addr < (Vmuchar_t*)caddr ) /* in sbrk space */
 			return NIL(Void_t*);
 		else
+#endif
 		{	(void)munmap(caddr, csize);
 			return caddr;
 		}
@@ -1385,7 +1388,9 @@ __DEFINE__(Vmalloc_t*, Vmheap, &_Vmheap);
 __DEFINE__(Vmalloc_t*, Vmregion, &_Vmheap);
 __DEFINE__(Vmethod_t*, Vmbest, &_Vmbest);
 __DEFINE__(Vmdisc_t*,  Vmdcsystem, (Vmdisc_t*)(&_Vmdcsystem) );
+#if _mem_sbrk
 __DEFINE__(Vmdisc_t*,  Vmdcsbrk, (Vmdisc_t*)(&_Vmdcsystem) );
+#endif
 
 #ifdef NoF
 NoF(vmbest)