Fix bootstrap tools build on macOS after 02af91c52e71e8a0f47251e637c9687f35d45dd9

After changing the namespace.h header we need to provide _err on macOS, too.
Previously we used the system libc err*/warn*, but that does not provide
_err/_warn (which is used by other bootstrapped files from libc).
To fix this problem bootstrap err.c on macOS as well.

Fixes:		02af91c52 (Fix crossbuild bootstrap tools build with Clang 12)

tools/build/Makefile

@@ -158,6 +158,9 @@ INCS+=	 ${SRCTOP}/include/getopt.h
 # getcap.c is needed for cap_mkdb:
 .PATH: ${LIBC_SRCTOP}/gen
 SRCS+=	getcap.c
+# Glibc does not provide all err*/warn* functions, and for macOS we need the
+# alias with the extra underscore.
+SRCS+=	err.c
 # Add various libbc functions that are not available in glibc:
 SRCS+=	stringlist.c setmode.c
 SRCS+=	strtonum.c merge.c heapsort.c reallocf.c
@@ -172,8 +175,8 @@ SRCS+=	strlcpy.c strlcat.c strmode.c
 SRCS+=	fgetln_fallback.c fgetwln_fallback.c closefrom.c
 CFLAGS.closefrom.c+=	-DSTDC_HEADERS -DHAVE_SYS_DIR_H -DHAVE_DIRENT_H \
 	-DHAVE_DIRFD -DHAVE_SYSCONF
-# Provide warnc/errc/getprogname/setprograme
-SRCS+=	err.c progname.c
+# Provide getprogname/setprograme
+SRCS+=	progname.c
 # Stub implementations of fflagstostr/strtofflags
 SRCS+=	fflags.c
 .endif