Change CMake generated pkgconfig to match meson.

[BillyONeal]

In vcpkg we want to make sure that CMake and pkg-config customers get an experience that works, including transitively. In reviewing microsoft/vcpkg#52891 we discovered that the transitive libjpeg-turbo -> libspng .pc link is broken. In trying to fix that I ran into a transitive problem here.

libspng/CMakeLists.txt

Lines 103 to 107 in adc9439

	foreach(libname ${spng_TARGETS})
	configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/libspng.pc.in ${CMAKE_CURRENT_BINARY_DIR}/cmake/lib${libname}.pc @ONLY)
	install(FILES ${CMAKE_CURRENT_BINARY_DIR}/cmake/lib${libname}.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)
	endforeach()

installs libspng.pc and libspng_static.pc, but

libspng/meson.build

Lines 82 to 87 in adc9439

	pkg = import('pkgconfig')
	pkg.generate(spng_lib,
	extra_cflags : spng_args,
	description : 'PNG decoding and encoding library'
	)

installs spng.pc. This seems to be the one libjpeg-turbo is looking to be compatible with.

https://packages.debian.org/sid/amd64/libspng-dev/filelist Looks like Debian is using the meson build, given that they install /usr/lib/x86_64-linux-gnu/pkgconfig/spng.pc.

Also, meson detects whether the .pc should link with libm by probing for existence of the library:

libspng/meson.build

Line 42 in adc9439

m_dep = cc.find_library('m', required : false)

while CMake just assumes "Windows gets no libm, everyone else does". This change changes everything to match meson.

The problems discussed here and some of the suggested outcomes are from GPT 5.6 Sol.