[vcpkg_fixup_pkgconfig] skip checks on paths with spaces

[Neumann-A]

• skip checks on paths with spaces
• introduce VCPKG_SKIP_PKGCONFIG_CHECK to skip the pkg-config if set to true|on
• some additional string replacements which might make spaces in paths work if a native pkg-config is used.

closes #12689

[JackBoosY]

After internal discussion, we believe that because std::filesystem::current_path is case-insensitive, the case of VCPKG_*_DIR does not match the actual path, and this is by design.
So please change the path to lowercase before replacing the absolute path with the relative path.

[PhoebeHui]

pbc issue would be fixed via #12732

[Neumann-A]

@JackBoosY: I don't believe the case insensitive part since the output everything is clearly uppercase:

CreateProcessW("E:\vcpkg_cache\downloads\tools\cmake-3.17.2-windows\cmake-3.17.2-win32-x86\bin\cmake.exe" -DCURRENT_PORT_DIR=E:/VCPKG/test/vcpkg/scripts/detect_compiler -DCURRENT_BUILDTREES_DIR=E:/VCPKG/test/vcpkg/buildtrees/detect_compiler -DCURRENT_PACKAGES_DIR=E:/VCPKG/test/vcpkg/packages/detect_compiler_x64-windows-static -DCMD=BUILD -DTARGET_TRIPLET=x64-windows-static "-DTARGET_TRIPLET_FILE=E:\VCPKG\test\vcpkg\triplets\x64-windows-static.cmake" -DVCPKG_PLATFORM_TOOLSET=v142 -DDOWNLOADS=E:/vcpkg_cache/downloads -DVCPKG_CONCURRENCY=9 "-DGIT=C:/Program Files/Git/cmd/git.exe" -DVCPKG_ROOT_DIR=E:/VCPKG/test/vcpkg -DPACKAGES_DIR=E:/VCPKG/test/vcpkg/packages -DBUILDTREES_DIR=E:/VCPKG/test/vcpkg/buildtrees -D_VCPKG_INSTALLED_DIR=E:/VCPKG/test/vcpkg/installed -DDOWNLOADS=E:/vcpkg_cache/downloads -DVCPKG_MANIFEST_INSTALL=OFF -P "E:\VCPKG\test\vcpkg\scripts\ports.cmake")

The same is true for a WSL build.

[Neumann-A]

So there is a trick to make spaces work which is basically to transform -L${libdir} etc into -L"${libdir}" and probably do the same for -I<somevar> (although that does not affect the script). The only problem I have is that the regex replace (line 147-153) removing the -L flags does not work any longer although if I print the string they should be equal. So CMake seems to hide some invisible characters which I couldn't figure out yet to correct the REGEX

[JackBoosY]

@Neumann-A It is always difficult to use cmake to deal with delimiters and quotes.