[Buildroot] [PATCH] package/libcap: fix build of host variant on systems without static libc

[Thomas Petazzoni <thomas.petazzoni@bootlin.com>]

Since the bump to 2.42 in 241e3bdea8b68b3ce78b1b76db29019fdcbb7b05,
the host variant of libcap fails to build on systems that don't have
the static variant of the C library installed. Indeed, libcap tries to
build test programs statically, which fails with:

/usr/bin/ld: cannot find -lc
collect2: error: ld returned 1 exit status

This commit fixes that by:

 (1) Adding a patch, submitted upstream, which makes the
     tests/Makefile obey the same DYNAMIC variable as the one used by
     progs/Makefile.

 (2) Explicitly enable DYNAMIC in libcap.mk. It might seems strange to
     unconditionally enable it, but in fact what DYNAMIC=yes does is
     to disable the use of --static. So in practice in Buildroot, this
     means that the libcap build system will rely on the LDFLAGS
     passed by Buildroot to decide if static or dynamic linking should
     be done, which is good.

Fixes:

  http://autobuild.buildroot.net/results/5ddca8a9d1e2c6353b628fbe8d65b440bfd4c440/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
---
 ...on-t-build-statically-if-DYNAMIC-yes.patch | 58 +++++++++++++++++++
 package/libcap/libcap.mk                      |  6 +-
 2 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 package/libcap/0001-tests-don-t-build-statically-if-DYNAMIC-yes.patch

diff --git a/package/libcap/0001-tests-don-t-build-statically-if-DYNAMIC-yes.patch b/package/libcap/0001-tests-don-t-build-statically-if-DYNAMIC-yes.patch
new file mode 100644
index 0000000000..3483826899
--- /dev/null
+++ b/package/libcap/0001-tests-don-t-build-statically-if-DYNAMIC-yes.patch
@@ -0,0 +1,58 @@
+From 8c11d901010265e4214f29eeded27f64746d90d9 Mon Sep 17 00:00:00 2001
+From: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
+Date: Fri, 4 Sep 2020 16:55:30 +0200
+Subject: [PATCH] tests: don't build statically if DYNAMIC=yes
+
+This takes the same approach as the one used in progs/Makefile.
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
+---
+ tests/Makefile | 12 ++++++++----
+ 1 file changed, 8 insertions(+), 4 deletions(-)
+
+diff --git a/tests/Makefile b/tests/Makefile
+index 0c86ee8..5a01f78 100644
+--- a/tests/Makefile
++++ b/tests/Makefile
+@@ -5,6 +5,10 @@ topdir=$(shell pwd)/..
+ include ../Make.Rules
+ #
+ 
++ifneq ($(DYNAMIC),yes)
++LDFLAGS += --static
++endif
++
+ DEPS=../libcap/libcap.a ../libcap/libpsx.a
+ 
+ all: psx_test libcap_psx_test libcap_launch_test
+@@ -28,23 +32,23 @@ run_libcap_psx_test: libcap_psx_test
+ 	./libcap_psx_test
+ 
+ libcap_psx_test: libcap_psx_test.c $(DEPS)
+-	$(CC) $(CFLAGS) $(IPATH) $< -o $@ $(LIBCAPLIB) $(LIBPSXLIB) -Wl,-wrap,pthread_create --static
++	$(CC) $(CFLAGS) $(IPATH) $< -o $@ $(LIBCAPLIB) $(LIBPSXLIB) -Wl,-wrap,pthread_create $(LDFLAGS)
+ 
+ run_libcap_launch_test: libcap_launch_test libcap_psx_launch_test noop
+ 	sudo ./libcap_launch_test
+ 	sudo ./libcap_psx_launch_test
+ 
+ libcap_launch_test: libcap_launch_test.c $(DEPS)
+-	$(CC) $(CFLAGS) $(IPATH) $< -o $@ $(LIBCAPLIB) --static
++	$(CC) $(CFLAGS) $(IPATH) $< -o $@ $(LIBCAPLIB) $(LDFLAGS)
+ 
+ # this varies only slightly from the above insofar as it currently
+ # only links in the pthreads fork support. TODO() we need to change
+ # the source to do something interesting with pthreads.
+ libcap_psx_launch_test: libcap_launch_test.c $(DEPS)
+-	$(CC) $(CFLAGS) $(IPATH) -DWITH_PTHREADS $< -o $@ $(LIBCAPLIB) $(LIBPSXLIB) -Wl,-wrap,pthread_create --static
++	$(CC) $(CFLAGS) $(IPATH) -DWITH_PTHREADS $< -o $@ $(LIBCAPLIB) $(LIBPSXLIB) -Wl,-wrap,pthread_create $(LDFLAGS)
+ 
+ noop: noop.c
+-	$(CC) $(CFLAGS) $< -o $@ --static
++	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
+ 
+ clean:
+ 	rm -f psx_test libcap_psx_test libcap_launch_test *~
+-- 
+2.26.2
+
diff --git a/package/libcap/libcap.mk b/package/libcap/libcap.mk
index 08c1bc9deb..6a35076f6e 100644
--- a/package/libcap/libcap.mk
+++ b/package/libcap/libcap.mk
@@ -26,9 +26,12 @@ LIBCAP_MAKE_TARGET = all
 LIBCAP_MAKE_INSTALL_TARGET = install
 endif
 
+# DYNAMIC=yes doesn't really mean "dynamic", it means "don't pass
+# --static". So we let the default Buildroot LDFLAGS do their job.
 LIBCAP_MAKE_FLAGS = \
 	BUILD_CC="$(HOSTCC)" \
-	BUILD_CFLAGS="$(HOST_CFLAGS)"
+	BUILD_CFLAGS="$(HOST_CFLAGS)" \
+	DYNAMIC=yes
 
 ifeq ($(BR2_PACKAGE_LIBCAP_TOOLS),y)
 define LIBCAP_BUILD_TOOLS_CMDS
@@ -62,6 +65,7 @@ endef
 
 define HOST_LIBCAP_BUILD_CMDS
 	$(HOST_MAKE_ENV) $(HOST_CONFIGURE_OPTS) $(MAKE) -C $(@D)\
+		DYNAMIC=yes \
 		RAISE_SETFCAP=no
 endef
 
-- 
2.26.2