Fix the build with slibtool.

[orbea]

When building libspiro with slibtool (https://dev.midipix.org/cross/slibtool) it fails in two places.

rdlibtool --tag=CC --mode=link gcc -Wall -Wextra -Wcast-align -Wbad-function-cast -Wc++-compat -Wmissing-prototypes -Wunused -Wdeprecated-declarations -g -O2 -Wall -Wextra -Wcast-align -Wbad-function-cast -Wc++-compat -Wmissing-prototypes -Wunused -Wdeprecated-declarations -no-undefined --mode=link -version-info 1:1:0 -o libspiro.la -rpath /usr/local/lib spiro.lo bezctx.lo spiroentrypoints.lo -lm

rdlibtool: lconf: {.name="libtool"}.
rdlibtool: fdcwd: {.fdcwd=AT_FDCWD, .realpath="/tmp/libspiro"}.
rdlibtool: lconf: fstatat(AT_FDCWD,".",...) = 0 {.st_dev = 45, .st_ino = 196675}.
rdlibtool: lconf: openat(AT_FDCWD,"libtool",O_RDONLY,0) = 3.
rdlibtool: lconf: found "/tmp/libspiro/libtool".
rdlibtool: link: x86_64-pc-linux-gnu-ar crs .libs/libspiro.a .libs/spiro.o .libs/bezctx.o .libs/spiroentrypoints.o
rdlibtool: link: gcc .libs/spiro.o .libs/bezctx.o .libs/spiroentrypoints.o -Wall -Wextra -Wcast-align -Wbad-function-cast -Wc++-compat -Wmissing-prototypes -Wunused -Wdeprecated-declarations -g -O2 -Wall -Wextra -Wcast-align -Wbad-function-cast -Wc++-compat -Wmissing-prototypes -Wunused -Wdeprecated-declarations --mode=link -lm -shared -fPIC -Wl,--no-undefined -Wl,-soname -Wl,libspiro.so.1 -o .libs/libspiro.so.1.0.1
gcc: error: unrecognized command-line option ‘--mode=link’
rdlibtool: exec error upon slbt_exec_link_create_library(), line 1566: (see child process error messages).
rdlibtool: < returned to > slbt_exec_link(), line 1996.
make[1]: *** [Makefile:532: libspiro.la] Error 2
make[1]: Leaving directory '/tmp/libspiro'
make: *** [Makefile:672: install-recursive] Error 1

rdlibtool --tag=CC --mode=link gcc -I.. -I.. -lm -g -O2 -o call-test0 call-test0.o ../.libs/libspiro.la -lm

rdlibtool: lconf: {.name="libtool"}.
rdlibtool: fdcwd: {.fdcwd=AT_FDCWD, .realpath="/tmp/libspiro/tests"}.
rdlibtool: lconf: fstatat(AT_FDCWD,".",...) = 0 {.st_dev = 45, .st_ino = 196774}.
rdlibtool: lconf: openat(AT_FDCWD,"libtool",O_RDONLY,0) = -1 [ENOENT].
rdlibtool: lconf: openat(AT_FDCWD,"../",O_DIRECTORY,0) = 3.
rdlibtool: lconf: fstat(3,...) = 0 {.st_dev = 45, .st_ino = 196675}.
rdlibtool: lconf: openat(3,"libtool",O_RDONLY,0) = 4.
rdlibtool: lconf: found "/tmp/libspiro/libtool".
rdlibtool: error logged in slbt_get_deps_meta(), line 126: path not found: ../.libs/.libs/libspiro.a.slibtool.deps.
rdlibtool: < returned to > slbt_exec_link_create_executable(), line 1617.
rdlibtool: < returned to > slbt_exec_link(), line 2062.
make[1]: *** [Makefile:718: call-test0] Error 2
make[1]: Leaving directory '/tmp/libspiro/tests'
make: *** [Makefile:675: install-recursive] Error 1

This reveals four issues:

1. The build should not use --mode=link unless its also calling $(LIBTOOL) manually which its not. GNU libtool silently consumes and ignores invalid command-line arguments hiding the issue.
2. The build should link dependencies with LIBADD for libraries or LDADD for programs while LDFLAGS is for other linker arguments. GNU libtool is far more permissive about these things than slibtool and hides incorrect usage.
3. The header dependencies should be in SOURCES and the build should not define object files rules manually.
4. The .libs directory should not be manually added to the linker paths, that is for internal $(LIBTOOL) usage.

Also please see this related PR fontforge/libuninameslist#24.

[orbea]

Also, just so you know, GNU libtool silently ignores -no-undefined while slibtool does not, its a common way programs fail. Thankfully it seems libspiro doesn't have hidden undefined references. :)

[JoesCat]

Thanks for taking the time to check and fix this to work with slibtool