configure.ac: Require C99

We already added AC_PROG_CC_C99 in r66605.
This commit stops warning C99 features.

[Misc #15347] [close https://github.com/ruby/ruby/pull/2064]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66771 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.ac

@@ -481,7 +481,6 @@ AS_CASE(["$GCC:${warnflags+set}:${extra_warnflags:+set}:"],
 	particular_werror_flags=no
     ])
     for wflag in \
-		 -Werror=declaration-after-statement \
 		 -Werror=deprecated-declarations \
 		 -Werror=division-by-zero \
 		 -Werror=duplicated-cond \
@@ -489,7 +488,6 @@ AS_CASE(["$GCC:${warnflags+set}:${extra_warnflags:+set}:"],
 		 -Werror=implicit-int \
 		 -Werror=misleading-indentation \
 		 -Werror=pointer-arith \
-		 -Werror=restrict \
 		 -Werror=shorten-64-to-32 \
 		 -Werror=write-strings \
 		 -Wimplicit-fallthrough=0 \