makefiles: expand HDRS, SRCS and OBJS immediately

Use immediate expansion of the right-hand side (with `:=`) to set the
variables to the output of the commands rather than to the (text of the)
commands themselves.

This should prevent deferred/lazy evaluation, which is something that
might potentially result in the relevant files being looked up each time
that HDRS and SRCS are evaluated.

Commands used to search and replace:

    git grep -Ilz '^SRCS' -- src | xargs -0 -I '{}' \
        sh -c "printf '%s\n' \"\$(sed \
            -e 's/^HDRS =/HDRS :=/' \
            -e 's/^SRCS =/SRCS :=/' \
            -e 's/^OBJS =/OBJS :=/' '{}')\" >'{}'"

1 files changed, 3 insertions, 3 deletions

diff --git a/src/common.mk b/src/common.mk
index 07082e183..c20aaf52f 100644
--- a/src/common.mk
+++ b/src/common.mk
@@ -2,9 +2,9 @@
 #
 # Note: $(ROOT)/config.mk must be included before this file.
 
-HDRS = $(sort $(wildcard *.h))
-SRCS = $(sort $(wildcard *.c))
-OBJS = $(SRCS:.c=.o)
+HDRS := $(sort $(wildcard *.h))
+SRCS := $(sort $(wildcard *.c))
+OBJS := $(SRCS:.c=.o)
 
 CFLAGS += -ggdb $(HAVE_FATAL_WARNINGS) -O2 -DVERSION='"$(VERSION)"' $(HAVE_GCOV)
 CFLAGS += -DPREFIX='"$(prefix)"' -DSYSCONFDIR='"$(sysconfdir)/firejail"' -DLIBDIR='"$(libdir)"' -DBINDIR='"$(bindir)"'   -DVARDIR='"/var/lib/firejail"'