Configure: support a few more "make variables" defaulting from env

CFLAGS, CXXFLAGS, CPPFLAGS, LDFLAGS, and LDLIBS

(cherry picked from commit 8e7984e5783877c58cddc7b4e668401580ab4467)
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8359)

Configure

@@ -550,18 +550,18 @@ my %user = (
     AS          => undef,
     ASFLAGS     => [],
     CC          => env('CC'),
-    CFLAGS      => [],
+    CFLAGS      => [ env('CFLAGS') || () ],
     CXX         => env('CXX'),
-    CXXFLAGS    => [],
+    CXXFLAGS    => [ env('CXXFLAGS') || () ],
     CPP         => undef,
-    CPPFLAGS    => [],  # -D, -I, -Wp,
+    CPPFLAGS    => [ env('CPPFLAGS') || () ],  # -D, -I, -Wp,
     CPPDEFINES  => [],  # Alternative for -D
     CPPINCLUDES => [],  # Alternative for -I
     CROSS_COMPILE => env('CROSS_COMPILE'),
     HASHBANGPERL=> env('HASHBANGPERL') || env('PERL'),
     LD          => undef,
-    LDFLAGS     => [],  # -L, -Wl,
-    LDLIBS      => [],  # -l
+    LDFLAGS     => [ env('LDFLAGS') || () ],  # -L, -Wl,
+    LDLIBS      => [ env('LDLIBS') || () ],  # -l
     MT          => undef,
     MTFLAGS     => [],
     PERL        => env('PERL') || ($^O ne "VMS" ? $^X : "perl"),