prereq-build: replace the openssl command check with a check for headers

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 42995

include/prereq-build.mk

@@ -168,8 +168,13 @@ $(eval $(call RequireCommand,svn, \
 	Please install the subversion client. \
 ))
 
-$(eval $(call RequireCommand,openssl, \
-	Please install openssl. \
+define Require/openssl
+	echo 'int main(int argc, char **argv) { SSL_library_init(); return 0; }' | \
+		gcc -include openssl/ssl.h -x c -o $(TMP_DIR)/a.out - -lcrypto -lssl
+endef
+
+$(eval $(call Require,openssl, \
+	Please install openssl (with development headers) \
 ))
 
 define Require/gnu-find