If you pass CFLAGS with a leading space, " " gets passed to popen and convinces
gcc to try and open a file called " ". This results in a confusing error message
like:

x86_64-pokysdk-linux-gcc: error: : No such file or directory

Avoid this by stripping empty elements out of CFLAGS.

Upstream-Status: Submitted [https://github.com/besser82/libxcrypt/pull/126]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Index: a/build-aux/compute-symver-floor
===================================================================
--- a/build-aux/compute-symver-floor
+++ b/build-aux/compute-symver-floor
@@ -36,6 +36,8 @@ sub preprocessor_check {
         die "C compiler not available\n" unless @CC;
 
         @CFLAGS = sh_split($ENV{CFLAGS} // q{});
+        # Remove empty elements, particularly leading ones which cause issues with popen below
+        @CFLAGS = grep {$_} @CFLAGS;
 
         # We call ensure_C_locale here, not from the main section,
         # because this sub might not get called at all, in which