BuildSystem: Allow own CFLAGS on ./configure.

Before this change, make failed with the error
Unknown value '' found in build_tools/menuselect-deps for NATIVE_ARCH
when CFLAGS were supplied to the configure script. This was introduced with
<https://reviewboard.asterisk.org/r/1852/> which disabled BUILD_NATIVE when
CFLAGS were supplied. Those who need different -march= values, please, go for
./configure
make menuselect.makeopts or make menuselect
./menuselect/menuselect --disable BUILD_NATIVE

ASTERISK-25289 #close

Change-Id: Ic6365d5a97bb9b3556858f06432a8d1cfa83eebc

configure

@@ -17756,19 +17756,13 @@ fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for -march=native support" >&5
 $as_echo_n "checking for -march=native support... " >&6; }
 if $(${CC} -march=native -S -o /dev/null -xc /dev/null > /dev/null 2>&1); then
-	if test "${CONFIG_CFLAGS}" = ""; then
-		{ $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
-		AST_NATIVE_ARCH=1
-	else
-		{ $as_echo "$as_me:${as_lineno-$LINENO}: result: user CFLAGS present" >&5
-$as_echo "user CFLAGS present" >&6; }
-		AST_NATIVE_ARCH=
-	fi
+	AST_NATIVE_ARCH=1
 else
 	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
-	AST_NATIVE_ARCH=
+	AST_NATIVE_ARCH=0
 fi
 
 

configure.ac

@@ -1213,16 +1213,11 @@ AC_SUBST(AST_SHADOW_WARNINGS)
 
 AC_MSG_CHECKING(for -march=native support)
 if $(${CC} -march=native -S -o /dev/null -xc /dev/null > /dev/null 2>&1); then
-	if test "${CONFIG_CFLAGS}" = ""; then
-		AC_MSG_RESULT(yes)
-		AST_NATIVE_ARCH=1
-	else
-		AC_MSG_RESULT(user CFLAGS present)
-		AST_NATIVE_ARCH=
-	fi
+	AC_MSG_RESULT(yes)
+	AST_NATIVE_ARCH=1
 else
 	AC_MSG_RESULT(no)
-	AST_NATIVE_ARCH=
+	AST_NATIVE_ARCH=0
 fi
 AC_SUBST(AST_NATIVE_ARCH)