sys: Stop enabling -Wnested-externs.

clang doesn't implement this warning, so violations are only caught by
GCC.  It is also no longer a common practice to use this as it was in
the original BSD code, so the need for the warning is not as important
as when it was used to do cleanups 20 years ago.  A recent commit
(c3179891f8) triggers this warning on
GCC, but that commit uses nested externs purposefully.

Reviewed by:	markj, emaste
Differential Revision:	https://reviews.freebsd.org/D39214

sys/conf/kern.mk

@@ -3,7 +3,7 @@
 #
 # Warning flags for compiling the kernel and components of the kernel:
 #
-CWARNFLAGS?=	-Wall -Wnested-externs -Wstrict-prototypes \
+CWARNFLAGS?=	-Wall -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Wcast-qual \
 		-Wundef -Wno-pointer-sign ${FORMAT_EXTENSIONS} \
 		-Wmissing-include-dirs -fdiagnostics-show-option \

sys/conf/kern.pre.mk

@@ -225,7 +225,6 @@ CDDL_CFLAGS=	\
 	-Wno-duplicate-decl-specifier \
 	-Wno-missing-braces \
 	-Wno-missing-prototypes \
-	-Wno-nested-externs \
 	-Wno-parentheses \
 	-Wno-pointer-arith \
 	-Wno-strict-prototypes \

sys/conf/kmod.mk

@@ -551,8 +551,7 @@ OPENZFS_CFLAGS=     \
 	-I${SYSDIR}/cddl/compat/opensolaris \
 	-I${SYSDIR}/cddl/contrib/opensolaris/uts/common \
 	-include ${ZINCDIR}/os/freebsd/spl/sys/ccompile.h
-OPENZFS_CWARNFLAGS= \
-	-Wno-nested-externs
+OPENZFS_CWARNFLAGS=
 
 .include <bsd.dep.mk>
 .include <bsd.clang-analyze.mk>

sys/modules/pms/Makefile

@@ -22,7 +22,6 @@ CFLAGS+=-fms-extensions
 
 CFLAGS+=-Wredundant-decls
 CFLAGS+=-Wunused-variable
-CFLAGS+=-Wnested-externs
 CFLAGS+=-Woverflow
 CFLAGS+=-Wmissing-prototypes
 CFLAGS+=-Wparentheses