Perl: propagated compile options from perl build.

Some Perl compile options affects ABI and not using them while compiling
our module resulted in non-working build.

Notably on 32-bit Debian 10, Perl is built with -D_FILE_OFFSET_BITS=64
and our module after being compiled without this option caused segmentation
faults in unexpected places.

auto/modules/perl

@@ -57,6 +57,7 @@ nxt_found=no
 if /bin/sh -c "$NXT_PERL -MConfig -e 'print \"Perl version: \",
     \$Config{version}, \"\\n\"'" >> $NXT_AUTOCONF_ERR 2>&1; then
 
+    NXT_PERL_CFLAGS=`$NXT_PERL -MExtUtils::Embed -e ccflags | sed -e 's/^ //;s/ $//'`
     NXT_PERL_INCLUDE=`$NXT_PERL -MExtUtils::Embed -e perl_inc | sed -e 's/^ //;s/ $//'`
     NXT_PERL_LDOPTS=`$NXT_PERL -MExtUtils::Embed -e ldopts | sed -e 's/^ //;s/ $//'`
 
@@ -165,7 +166,7 @@ for nxt_src in $NXT_PERL_MODULE_SRCS; do
 
 $NXT_BUILD_DIR/$nxt_obj:	$nxt_src
 	mkdir -p $NXT_BUILD_DIR/src/perl
-	\$(CC) -c \$(CFLAGS) \$(NXT_INCS) $NXT_PERL_INCLUDE \\
+	\$(CC) -c \$(CFLAGS) $NXT_PERL_CFLAGS \$(NXT_INCS) $NXT_PERL_INCLUDE \\
 	$nxt_dep_flags \\
 	-o $NXT_BUILD_DIR/$nxt_obj $nxt_src
 	$nxt_dep_post

src/perl/nxt_perl_psgi_layer.h

@@ -8,7 +8,6 @@
 #define _NXT_PERL_PSGI_LAYER_H_INCLUDED_
 
 
-#define _GNU_SOURCE
 #include <EXTERN.h>
 #include <XSUB.h>
 #include <perl.h>