Enable link-time optimization for central libraries with clang.

configure.ac

@@ -237,16 +237,25 @@ dnl Explicit link-time-optimization switch
 dnl
 
 AC_MSG_CHECKING(whether to enable link-time optimization)
-AC_ARG_ENABLE(lto, AC_HELP_STRING([--enable-lto], [Enable link-time optimization]),
+AC_ARG_ENABLE(lto, AC_HELP_STRING([--disable-lto], [Enable link-time optimization]),
 [
-	AC_MSG_RESULT([yes])
+	lto="no"
-	AC_SUBST(LTO, 1)
+	AC_MSG_RESULT([no, explicitly disabled])
 ], [
-	AM_COND_IF(DEBUG,
+	AM_COND_IF(OPTIMIZE,
 	[
-		AC_MSG_RESULT([no])
+		AM_COND_IF(CLANG,
+		[
+			lto="yes"
+			AC_MSG_RESULT([yes])
+			AC_SUBST(LTO, 1)
+		], [
+			lto="no"
+			AC_MSG_RESULT([no, only available with clang])
+		])
 	], [
-		AC_MSG_RESULT([no, link-time optimization is explicit-only at this time])
+		lto="no"
+		AC_MSG_RESULT([no, optimizations disabled])
 	])
 ])
 
@@ -511,14 +520,6 @@ dnl
 
 CXXFLAGS+=" -O$optimize_level"
 
-AM_COND_IF([LTO],
-[
-	lto="yes"
-	CXXFLAGS+=" -flto=`nproc`"
-], [
-	lto="no"
-])
-
 AM_COND_IF([OPTIMIZE],
 [
 	optimize="yes"

ircd/Makefile.am

@@ -27,6 +27,10 @@ AM_LDFLAGS = \
 	@EXTRA_LDFLAGS@ \
 	###
 
+if LTO
+AM_CXXFLAGS += -flto
+endif
+
 if DEBUG
 if GCC
 AM_CXXFLAGS += -fmax-errors=2

matrix/Makefile.am

@@ -35,6 +35,10 @@ if GCC
 AM_CXXFLAGS += -ftls-model=local-dynamic
 endif
 
+if LTO
+AM_CXXFLAGS += -flto
+endif
+
 if DEBUG
 if GCC
 AM_CXXFLAGS += -fmax-errors=3