configure.in: Changes empty CFLAGS check to be (more) shell neutral.
The test for empty CFLAGS used the bash (and compatible) -z test to check
for a zero length string.
This changes it to the more shell neutral "x$foo" = "x" form that's also
used elsewhere in this file.
The AS_IF macro is the preferred way of expressing these tests.
Since there are a few of them in this file, such a rewrite would touch
large parts of it. Perhaps something to look into later.
diff --git a/configure.in b/configure.in
index 7684445..f341048 100644
--- a/configure.in
+++ b/configure.in
@@ -2,7 +2,7 @@
AC_INIT([libsrtp2], [2.1.0-pre], [https://github.com/cisco/libsrtp/issues])
dnl Must come before AC_PROG_CC
-if test -z "$CFLAGS"; then
+if test "x$CFLAGS" = "x"; then
dnl Default value for CFLAGS if not specified.
CFLAGS="-Wall -pedantic -O4 -fexpensive-optimizations -funroll-loops"
fi