Make link time optimization flags configurable

We want to use -flto=auto in Arch Linux to speed up building, but we
can't hardcode it in buildenv/lto.sh because other downstreams might
have clang < 13.0.0 which did not recognize -flto=auto as equivalent
to -flto=full.

Introducing an LTOFLAGS variable to makepkg.conf seems the way to go.
Signed-off-by: Allan McRae <allan@archlinux.org>

doc/makepkg.conf.5.asciidoc

@@ -81,6 +81,11 @@ Options
 	usage resembling ``-Wl,--hash-style=gnu''. Read ld(1) for more details on
 	available linker flags.
 
+**LTOFLAGS=**"ltoflags"::
+	Additional compiler and linker flags appended to `CFLAGS`, `CXXFLAGS`
+	and `LDFLAGS` when building with link time optimization. If empty,
+	``-flto'' is used.
+
 **MAKEFLAGS=**"makeflags"::
 	This is often used to set the number of jobs used; for example, `-j2`.
 	Other flags that make accepts can also be passed.
@@ -190,8 +195,9 @@ Options
 		package containing the debug symbols when used with `strip'.
 
 	*lto*;;
-		Enable building packages using link time optimization. Adds '-flto'
-		to both CFLAGS and CXXFLAGS.
+		Enable building packages using link time optimization. Adds the
+		flags specified in LTOFLAGS to CFLAGS, CXXFLAGS and LDFLAGS (or
+		``-flto'' if LTOFLAGS is empty).
 
 	*autodep*;;
 		Enable the automatic addition of libraries to the depends and

etc/makepkg.conf.in

@@ -41,6 +41,7 @@ CHOST="@CHOST@"
 #CFLAGS="-O2 -pipe"
 #CXXFLAGS="-O2 -pipe"
 #LDFLAGS=""
+#LTOFLAGS="-flto"
 #RUSTFLAGS="-C opt-level=2"
 #-- Make Flags: change this for DistCC/SMP systems
 #MAKEFLAGS="-j2"

scripts/libmakepkg/buildenv/buildflags.sh.in

@@ -30,6 +30,6 @@ buildenv_functions+=('buildenv_buildflags')
 
 buildenv_buildflags() {
 	if check_option "buildflags" "n"; then
-		unset CPPFLAGS CFLAGS DEBUG_CFLAGS CXXFLAGS DEBUG_CXXFLAGS LDFLAGS RUSTFLAGS DEBUG_RUSTFLAGS
+		unset CPPFLAGS CFLAGS DEBUG_CFLAGS CXXFLAGS DEBUG_CXXFLAGS LDFLAGS LTOFLAGS RUSTFLAGS DEBUG_RUSTFLAGS
 	fi
 }

scripts/libmakepkg/buildenv/lto.sh.in

@@ -31,8 +31,8 @@ buildenv_functions+=('buildenv_lto')
 
 buildenv_lto() {
 	if check_option "lto" "y" && ! check_option "buildflags" "n"; then
-		CFLAGS+=" -flto"
-		CXXFLAGS+=" -flto"
-		LDFLAGS+=" -flto"
+		CFLAGS+=" ${LTOFLAGS:--flto}"
+		CXXFLAGS+=" ${LTOFLAGS:--flto}"
+		LDFLAGS+=" ${LTOFLAGS:--flto}"
 	fi
 }

scripts/libmakepkg/lint_config/variable.sh.in

@@ -32,10 +32,10 @@ lint_config_variables() {
 	local array=(DLAGENTS VCSCLIENTS BUILDENV OPTIONS INTEGRITY_CHECK MAN_DIRS
 	             DOC_DIRS PURGE_TARGETS COMPRESSGZ COMPRESSBZ2 COMPRESSXZ
 	             COMPRESSLRZ COMPRESSLZO COMPRESSZ)
-	local string=(CARCH CHOST CPPFLAGS CFLAGS CXXFLAGS RUSTFLAGS LDFLAGS DEBUG_CFLAGS
-	              DEBUG_CXXFLAGS DEBUG_RUSTFLAGS DISTCC_HOSTS BUILDDIR STRIP_BINARIES
-	              STRIP_SHARED STRIP_STATIC PKGDEST SRCDEST SRCPKGDEST LOGDEST PACKAGER
-	              GPGKEY PKGEXT SRCEXT)
+	local string=(CARCH CHOST CPPFLAGS CFLAGS CXXFLAGS RUSTFLAGS LDFLAGS LTOFLAGS
+	              DEBUG_CFLAGS DEBUG_CXXFLAGS DEBUG_RUSTFLAGS DISTCC_HOSTS BUILDDIR
+	              STRIP_BINARIES STRIP_SHARED STRIP_STATIC PKGDEST SRCDEST SRCPKGDEST
+	              LOGDEST PACKAGER GPGKEY PKGEXT SRCEXT)
 
 	local i keys ret=0