configure. In configure builds 'make install' changes/adds links like png.h
and libpng.a to point to the newly installed, versioned, files (e.g.
libpng17/png.h and libpng17.a). Three new configure options and some
rearrangement of Makefile.am allow creation of these links to be disabled.
1) Added the 'compile' program to the list of programs that must be cleaned
in autogen.sh
2) Added 'subdir-objects' which causes .c files in sub-directories to be
compiled such that the corresponding .o files are also in the
sub-directory. This is because automake 1.14 warns that the
current behavior of compiling to the top level directory may be removed
in the future.
3) Updated dependencies on pnglibconf.h to match the new .o locations and
added all the files in contrib/libtests and contrib/tools that depend
on pnglibconf.h
4) Added 'BUILD_SOURCES = pnglibconf.h'; this is the automake recommended
way of handling the dependencies of sources that are machine generated;
unfortunately it only works if the user does 'make all' or 'make check',
so the dependencies (3) are still required.
prefixing. This adds an API to set optimization options externally,
providing an alternative and general solution for the non-portable
run-time tests used by the ARM Neon code. It also makes those tests
compile and link on Android. The order of settings vs options in
pnglibconf.h is reversed to allow settings to depend on options and
options can now set (or override) the defaults for settings.
Allow run-time ARM NEON checking to be disabled. A new configure option:
--enable-arm-neon=always will stop the run-time checks. New checks
within arm/arm_init.c will cause the code not to be compiled unless
__ARM_NEON__ is set. This should make it fail safe (if someone asks
for it on then the build will fail if it can't be done.)
