add missing POSIX confstr keys for pthread CFLAGS/LDFLAGS

_CS_POSIX_V7_THREADS_CFLAGS and _CS_POSIX_V7_THREADS_LDFLAGS have been
missing for a long time, which is a conformance defect. we were
waiting on glibc to add them or at least agree on the numeric values
they will have so as to keep the numbering aligned. it looks like they
will be added to glibc with these numbers, and in any case, this list
does not have any significant churn that would result in the numbers
getting taken.

include/unistd.h

@@ -467,6 +467,8 @@ pid_t gettid(void);
 #define _CS_POSIX_V7_LPBIG_OFFBIG_LINTFLAGS	1147
 #define _CS_V6_ENV	1148
 #define _CS_V7_ENV	1149
+#define _CS_POSIX_V7_THREADS_CFLAGS	1150
+#define _CS_POSIX_V7_THREADS_LDFLAGS	1151
 
 #ifdef __cplusplus
 }

src/conf/confstr.c

@@ -7,7 +7,7 @@ size_t confstr(int name, char *buf, size_t len)
 	const char *s = "";
 	if (!name) {
 		s = "/bin:/usr/bin";
-	} else if ((name&~4U)!=1 && name-_CS_POSIX_V6_ILP32_OFF32_CFLAGS>33U) {
+	} else if ((name&~4U)!=1 && name-_CS_POSIX_V6_ILP32_OFF32_CFLAGS>35U) {
 		errno = EINVAL;
 		return 0;
 	}