package/proftpd: needs dynamic library

proftpd doesn't build statically because it does not use pkg-config to
retrieve its dependencies since its addition in commit
5d173ec4122b89857e6d93fc337311b1f87631c9:

/tmp/instance-13/output-1/host/lib/gcc/arc-buildroot-linux-uclibc/10.2.0/../../../../arc-buildroot-linux-uclibc/bin/ld: /tmp/instance-13/output-1/host/arc-buildroot-linux-uclibc/sysroot/usr/lib/libidn2.a(libunistring_la-striconveh.o): in function `iconv_carefully_1':
buildroot/build/libidn2-2.3.4/unistring/striconveh.c:233: undefined reference to `libiconv'

[...]

/tmp/instance-5/output-1/host/lib/gcc/arceb-buildroot-linux-uclibc/10.2.0/../../../../arceb-buildroot-linux-uclibc/bin/ld: /tmp/instance-5/output-1/host/arceb-buildroot-linux-uclibc/sysroot/usr/lib/libcrypto.a(bio_lib.o): in function `BIO_free':
bio_lib.c:(.text+0x3f2): undefined reference to `__atomic_fetch_sub_4'

Fixes:
 - http://autobuild.buildroot.org/results/09f3f37b63dc68d31b18816b818df86fa3be095c
 - http://autobuild.buildroot.org/results/2aba5297206e44dc086c3138ace70e85739196f3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/proftpd/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_PROFTPD
 	bool "proftpd"
 	depends on BR2_USE_MMU # fork()
+	depends on !BR2_STATIC_LIBS
 	help
 	  ProFTPD, a highly configurable FTP server.
 
@@ -104,3 +105,7 @@ config BR2_PACKAGE_PROFTPD_BUFFER_SIZE
 	  0 uses the default size of 1024.
 
 endif
+
+comment "proftpd needs a toolchain w/ dynamic library"
+	depends on BR2_USE_MMU
+	depends on BR2_STATIC_LIBS