build: ensure rules with pipes might fail

.SHELLFLAGS is extended by option pipefail to make pipes fail if any pipe
element fails. As .SHELLFLAGS is exported into sub-make instances it
must be unexported before calling third-party build systems recursively.

repos/base-pistachio/lib/mk/syscall-pistachio.mk

@@ -9,6 +9,9 @@ CC_WARN += -Wno-array-bounds -Wno-unused-but-set-variable \
            -Wno-parentheses -Wno-format -Wno-builtin-declaration-mismatch \
            -Wno-unused-function -Wno-pointer-compare
 
+# do not confuse third-party sub-makes
+unexport .SHELLFLAGS
+
 user_build.tag:
 	LIBGCCFLAGS="$(CC_MARCH)" \
 	LDFLAGS="$(addprefix $(LD_PREFIX),$(LD_MARCH)) -nostdlib" \

repos/base-pistachio/src/kernel/pistachio/target.mk

@@ -24,6 +24,9 @@ $(KERNEL_BUILD_DIR)/Makefile:
 #
 unexport CCACHE
 
+# do not confuse third-party sub-makes
+unexport .SHELLFLAGS
+
 #
 # How to pass custom compiler flags to the Pistachio build system
 #