build system: detect missing crypt and drop it from linking

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

Makefile.flags

@@ -97,13 +97,6 @@ CFLAGS += $(strip $(subst ",,$(CONFIG_EXTRA_CFLAGS)))
 #"))
 endif
 
-ifneq ($(CONFIG_CROSS_COMPILER_PREFIX),"arm-linux-androideabi-")
-LDLIBS += m crypt
-else
-# Android libc has no crypt. TODO: make a generic CONFIG_LINK_WITH_CRYPT option?
-LDLIBS += m
-endif
-
 # Note: both "" (string consisting of two quote chars) and empty string
 # are possible, and should be skipped below.
 ifneq ($(subst "",,$(CONFIG_SYSROOT)),)
@@ -111,6 +104,14 @@ CFLAGS += --sysroot=$(CONFIG_SYSROOT)
 export SYSROOT=$(CONFIG_SYSROOT)
 endif
 
+# Android has no separate crypt library
+CRYPT_AVAILABLE := $(shell echo 'int main(void){return 0;}' | $(CC) $(CFLAGS) $(1) -lcrypt -o /dev/null -xc - >/dev/null 2>&1 && echo "y")
+ifeq ($(CRYPT_AVAILABLE),y)
+LDLIBS += m crypt
+else
+LDLIBS += m
+endif
+
 ifeq ($(CONFIG_PAM),y)
 # libpam uses libpthread, so for static builds busybox must be linked to
 # libpthread. On some platforms that requires an explicit -lpthread, so