libstdc++: Do not check getentropy and arc4random for cross builds

The "getentropy" and "arc4random" check may not yield a correct result
for the cross compile builds because linking is often not available for
them and the C library headers (notoriously, newlib) may still declare
these function prototypes even if they are not actually part of the
final library -- for this reason, this commit disables the "getentropy"
and "arc4random" checks for non-native builds.

This effectively prevents the std::random_device from making use of
these functions when `--with-newlib` is specified, which is indeed a
correct behaviour because the newlib does not provide a default stub
for the "getentropy" function (also note that the newlib "arc4random"
implementation internally calls the missing "getentropy" function).

For other C libraries, the `GLIBCXX_CROSSCONFIG` function may hard-code
the availability of these functions by manually defining
`HAVE_GETENTROPY` and `HAVE_ARC4RANDOM`, or by calling the
`GLIBCXX_CHECK_GETENTROPY` and `GLIBCXX_CHECK_ARC4RANDOM` functions.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

libstdc++-v3/configure.ac

@@ -269,6 +269,10 @@ if $GLIBCXX_IS_NATIVE; then
   # For /dev/random and /dev/urandom for std::random_device.
   GLIBCXX_CHECK_DEV_RANDOM
 
+  # Check for other random number APIs
+  GLIBCXX_CHECK_GETENTROPY
+  GLIBCXX_CHECK_ARC4RANDOM
+
   # For TLS support.
   GCC_CHECK_TLS
 
@@ -474,10 +478,6 @@ GLIBCXX_CHECK_X86_RDRAND
 # Check if assembler supports rdseed opcode.
 GLIBCXX_CHECK_X86_RDSEED
 
-# Check for other random number APIs
-GLIBCXX_CHECK_GETENTROPY
-GLIBCXX_CHECK_ARC4RANDOM
-
 # This depends on GLIBCXX_ENABLE_SYMVERS and GLIBCXX_IS_NATIVE.
 GLIBCXX_CONFIGURE_TESTSUITE