configure: Allow mixing declarations and statements

This C90 rule forces us to use a too big scope and should
therefore be dropped. Given that we already require C11,
all supported compilers can handle mixed declarations
and statements just fine.

Reviewed-by: Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>
Reviewed-by: Marvin Scholz <epirat07@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Reviewed-by: softworkz . <softworkz-at-hotmail.com@ffmpeg.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

configure

@@ -7467,7 +7467,6 @@ fi
 enabled debug && add_cflags -g"$debuglevel" && add_asflags -g"$debuglevel"
 
 # add some useful compiler flags if supported
-check_cflags -Wdeclaration-after-statement
 check_cflags -Wall
 check_cflags -Wdisabled-optimization
 check_cflags -Wpointer-arith
@@ -7504,10 +7503,6 @@ check_disable_warning -Wno-bool-operation
 check_disable_warning -Wno-char-subscripts
 check_disable_warning -Wno-implicit-const-int-float-conversion
 
-# Disable mixed decl and code warning for Objective-C
-test_objcflags $unknown_warning_flags '-Wdeclaration-after-statement' &&
-    add_objcflags '-Wno-declaration-after-statement'
-
 check_disable_warning_headers(){
     warning_flag=-W${1#-Wno-}
     test_cflags $warning_flag && add_cflags_headers $1

doc/developer.texi

@@ -70,9 +70,6 @@ variable-length arrays;
 
 @item
 complex numbers;
-
-@item
-mixed statements and declarations.
 @end itemize
 
 @subsection SIMD/DSP