Fix: GCC ninja module pchheader generation

[ngorden]

What does this PR do?

• fixes Clang/GCC pchheader command to remove path duplication in generated build.ninja files

How does this PR change Premake's behavior?

• writes correct ninja rules on clang for Unix machines

Anything else we should know?

given a dummy project with the following structure:

$ tree
.
├── build.ninja
├── HostApp
│   ├── HostApp.ninja
│   ├── include
│   │   └── pch.h
│   ├── premake5.lua
│   └── src
│       └── main.cc
├── premake5.lua
├── provision.sh
├── README.md
├── run-tests
└── Vagrantfile

4 directories, 11 files
$ cat HostApp/premake5.lua 
project "HostApp"
    kind "ConsoleApp"
    language "C++"
    cppdialect "C++17"

    files { "src/*.cc" }
    includedirs { "include" }
    pchheader "pch.h"%   

premake v5.0.0-beta8 generated build.ninja:

$ cat HostApp/HostApp.ninja
...
build HostApp/obj/Debug/pch.h.gch | HostApp/obj/Debug/pch.h.gch.d: pch_clang HostApp/HostApp/include/pch.h # <- pch.h doesn't exist here
  cflags = $cxxflags_HostApp_Debug
  depfile = HostApp/obj/Debug/pch.h.gch.d
build HostApp/obj/Debug/main.o: cxx_clang HostApp/src/main.cc | HostApp/obj/Debug/pch.h.gch
  cxxflags = $cxxflags_HostApp_Debug -include obj/Debug/pch.h # <- pch.h doesn't exist here
build HostApp/bin/Debug/HostApp: link_clang HostApp/obj/Debug/main.o
  ldflags = $ldflags_HostApp_Debug

$ ninja
ninja: error: 'HostApp/HostApp/include/pch.h', needed by 'HostApp/obj/Debug/pch.h.gch', missing and no known rule to make it

post-changes output:

$ cat HostApp/HostApp.ninja
...
build HostApp/obj/Debug/pch.h.gch | HostApp/obj/Debug/pch.h.gch.d: pch_gcc HostApp/include/pch.h
  cflags = $cxxflags_HostApp_Debug
  depfile = HostApp/obj/Debug/pch.h.gch.d
build HostApp/obj/Debug/main.o: cxx_gcc HostApp/src/main.cc | HostApp/obj/Debug/pch.h.gch
  cxxflags = $cxxflags_HostApp_Debug -I HostApp/include -include HostApp/obj/Debug/pch.h
build HostApp/bin/Debug/HostApp: link_gcc HostApp/obj/Debug/main.o
  ldflags = $ldflags_HostApp_Debug

$ ninja
[1/3] Generating precompiled header HostApp/include/pch.h
[2/3] Compiling C++ source HostApp/src/main.cc
[3/3] Linking target HostApp/bin/Debug/HostApp

Did you check all the boxes?

• Focus on a single fix or feature; remove any unrelated formatting or code changes
• Add unit tests showing fix or feature works; all tests pass
• Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• Follow our coding conventions
• Minimize the number of commits
• Align documentation to your changes

[nickclark2016]

Verified these changes locally. Leaving open for ~24 hours for comments before merging. Thank you!