Build: Make package cfg files/install relocatable

This fully reverts c96e93b.

Specifying a full path with install(TARGETS ...) causes the paths in the
CMake package config files to be hard-coded, which effectively makes the
package non-relocatable.  Specifying a full path with other install()
commands breaks the --prefix option to cmake --install.  There is in
fact no sane way to support blank install directories.  Integrators who
wish to install certain files into the main libjpeg-turbo install
directory can set a particular CMAKE_INSTALL_*DIR variable to
"<CMAKE_INSTALL_PREFIX>", with the understanding that doing so will
break relocatability in the same way that
c96e93b did.

This commit also tweaks the RPM spec so that RPMs built from a source
RPM will contain relocatable CMake package config files.

Build: Make package cfg files/install relocatable

This fully reverts c96e93b.

Specifying a full path with install(TARGETS ...) causes the paths in the
CMake package config files to be hard-coded, which effectively makes the
package non-relocatable.  Specifying a full path with other install()
commands breaks the --prefix option to cmake --install.  There is in
fact no sane way to support blank install directories.  Integrators who
wish to install certain files into the main libjpeg-turbo install
directory can set a particular CMAKE_INSTALL_*DIR variable to
"<CMAKE_INSTALL_PREFIX>", with the understanding that doing so will
break relocatability in the same way that
c96e93b did.

This commit also tweaks the RPM spec so that RPMs built from a source
RPM will contain relocatable CMake package config files.

Pkg/RPM: Eliminate cmake-filesystem dependency

The Independent JPEG Group's JPEG software v10

Fix Visual C++ compiler warnings

(introduced by previous commit)

Remove vestigial filenames from SIMD code headers

These were a relic of libjpeg/SIMD, which attempted to follow the
conventions of the libjpeg source code, but they are no longer relevant
(or even accurate in some cases.)

Build: Don't modify CMAKE_C_FLAGS_* if specified

The build system originally force-enabled the maximum optimization level
(-O3) with GCC because it significantly improved the performance of the
C Huffman encoder on x86 platforms.  Since libjpeg-turbo 1.5.x, the
Huffman encoder has been SIMD-accelerated on x86 and Arm platforms, and
in my testing on various Intel CPUs, the difference between -O2 and -O3
is no longer statistically significant.  However, on certain Arm CPUs, I
observe that grayscale decompression slows down by 16-27% with -O2 vs.
-O3.  Although modern versions of CMake use -O3 as the default
optimization level for Release builds, -O2 is still the default
optimization level for RelWithDebInfo builds.  Thus, I am reluctant to
change the default behavior of our build system.  However, referring
to #815, some users observe better performance with -O2 vs. -O3 on other
Arm CPUs, so the build system needs to allow users to override the
default behavior.

Closes #815

Build: Fix float test errors with Xcode 14.2/Arm

Unlike other versions of Clang 14.0.0, AppleClang 14.0.0 in Xcode 14.2
retains the old -ffp-contract=off default.  Apple didn't adopt
-ffp-contract=on as the default until Xcode 14.3 (AppleClang 14.0.3.)
This has been confirmed in the Xcode 14.3 release notes.

Build: Fix typo

tj3Transform: Don't calc dst subsamp unless needed

This just improves code readability by emphasizing that we don't care
about the destination image's level of subsampling unless
TJPARAM_NOREALLOC is set or lossless cropping will be performed.