oslib-posix: check for posix_memalign in configure script

Check for the presence of posix_memalign() in the configure script,
not using "defined(_POSIX_C_SOURCE) && !defined(__sun__)". This
lets qemu use posix_memalign() on NetBSD versions that have it,
instead of falling back to valloc() which is wasteful when the
required alignment is smaller than a page.

Backports commit 9bc5a7193fb422ee53187601eba577ee5d195522 from qemu

qemu/configure

@@ -1196,6 +1196,21 @@ if test "$sanitizers" = "yes" ; then
   fi
 fi
 
+##########################################
+# check if we have posix_memalign()
+
+posix_memalign=no
+cat > $TMPC << EOF
+#include <stdlib.h>
+int main(void) {
+    void *p;
+    return posix_memalign(&p, 8, 8);
+}
+EOF
+if compile_prog "" "" ; then
+    posix_memalign=yes
+fi
+
 ##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs

qemu/util/oslib-posix.c

@@ -84,7 +84,7 @@ void *qemu_try_memalign(size_t alignment, size_t size)
         alignment = sizeof(void*);
     }
 
-#if defined(_POSIX_C_SOURCE) && !defined(__sun__)
+#if defined(CONFIG_POSIX_MEMALIGN)
     int ret;
     ret = posix_memalign(&ptr, alignment, size);
     if (ret != 0) {