autotools: enable -fdata/function-sections and --gc-sections

Analogue to the parent cmake commit, with linker flag detection.

The former two split the functions and data into separate sections
within the object file. Which makes it easier for the latter to properly
garbage collect and discard unused sections.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

Makefile.am

@@ -284,7 +284,7 @@ endif
 
 # -no-undefined marks that libarchive doesn't rely on symbols
 # defined in the application.  This is mandatory for cygwin.
-libarchive_la_LDFLAGS= -no-undefined -version-info $(ARCHIVE_LIBTOOL_VERSION)
+libarchive_la_LDFLAGS= -no-undefined -version-info $(ARCHIVE_LIBTOOL_VERSION) $(GC_SECTIONS)
 libarchive_la_LIBADD= $(LTLIBICONV)
 
 # Manpages to install
@@ -1023,7 +1023,7 @@ endif
 
 bsdtar_LDADD= libarchive.la libarchive_fe.la $(LTLIBICONV)
 bsdtar_CPPFLAGS= -I$(top_srcdir)/libarchive -I$(top_srcdir)/libarchive_fe $(bsdtar_ccstatic) $(PLATFORMCPPFLAGS)
-bsdtar_LDFLAGS= $(bsdtar_ldstatic)
+bsdtar_LDFLAGS= $(bsdtar_ldstatic) $(GC_SECTIONS)
 
 bsdtar_EXTRA_DIST= \
 	tar/bsdtar.1 \
@@ -1189,7 +1189,7 @@ endif
 
 bsdcpio_LDADD= libarchive_fe.la libarchive.la $(LTLIBICONV)
 bsdcpio_CPPFLAGS= -I$(top_srcdir)/libarchive -I$(top_srcdir)/libarchive_fe $(bsdcpio_ccstatic) $(PLATFORMCPPFLAGS)
-bsdcpio_LDFLAGS= $(bsdcpio_ldstatic)
+bsdcpio_LDFLAGS= $(bsdcpio_ldstatic) $(GC_SECTIONS)
 
 bsdcpio_EXTRA_DIST= \
 	cpio/bsdcpio.1 \
@@ -1343,7 +1343,7 @@ endif
 
 bsdcat_LDADD= libarchive_fe.la libarchive.la $(LTLIBICONV)
 bsdcat_CPPFLAGS= -I$(top_srcdir)/libarchive -I$(top_srcdir)/libarchive_fe $(bsdcat_ccstatic) $(PLATFORMCPPFLAGS)
-bsdcat_LDFLAGS= $(bsdcat_ldstatic)
+bsdcat_LDFLAGS= $(bsdcat_ldstatic) $(GC_SECTIONS)
 
 bsdcat_EXTRA_DIST= \
 	cat/bsdcat.1 \

configure.ac

@@ -540,6 +540,22 @@ fi
 # Checks for supported compiler flags
 AX_APPEND_COMPILE_FLAGS([-Wall -Wformat -Wformat-security])
 
+# Place the functions and data into separate sections, allowing the linker
+# to garbage collect the unused ones.
+save_LDFLAGS=$LDFLAGS
+LDFLAGS="$LDFLAGS -Wl,--gc-sections"
+AC_MSG_CHECKING([whether ld supports --gc-sections])
+AC_LINK_IFELSE(
+    [AC_LANG_SOURCE([static char UnusedFunc() { return 5; } int main() { return 0;}])],
+    [AC_MSG_RESULT([yes])
+        GC_SECTIONS="-Wl,--gc-sections";
+        AX_APPEND_COMPILE_FLAGS([-ffunction-sections -fdata-sections])],
+    [AC_MSG_RESULT([no])
+        GC_SECTIONS="";])
+LDFLAGS=$save_LDFLAGS
+
+AC_SUBST(GC_SECTIONS)
+
 # Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
 # la_TYPE_UID_T defaults to "int", which is incorrect for MinGW