Sanitize build system for packagers

The current build system is unusable for any packager because of the hard ties with its third parties.

The work of the packager is usually to build separate controlled packages for every unit. In the case of shaderc, there will be a package for spirv-headers, a package for spirv-tools, and a package for glslang. All of them will deploy all the necessary libs and headers in the appropriate prefix.

To build shaderc on top of that, I had to hack the build system. Here is a draft:

```diff
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0c8d1ae..f9fbf53 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -110,9 +110,13 @@ if(MSVC)
endif(MSVC)

+option(SHADERC_ENABLE_THIRD_PARTY "Pull third party dependencies" ON)
+
+if (SHADERC_ENABLE_THIRD_PARTY)
# Configure subdirectories.
# We depend on these for later projects, so they should come first.
add_subdirectory(third_party)
+endif()

add_subdirectory(libshaderc_util)
add_subdirectory(libshaderc)
@@ -121,11 +125,19 @@ if(${SHADERC_ENABLE_EXAMPLES})
add_subdirectory(examples)
endif()

+if (SHADERC_ENABLE_THIRD_PARTY)
add_custom_target(build-version
${PYTHON_EXECUTABLE}
${CMAKE_CURRENT_SOURCE_DIR}/utils/update_build_version.py
${shaderc_SOURCE_DIR} ${spirv-tools_SOURCE_DIR} ${glslang_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR}/build-version.inc
COMMENT "Update build-version.inc in the Shaderc build directory (if necessary).")
+else()
+ file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/build-version.inc
+ "\"shaderc TODO\\n\"\n"
+ "\"spirv-tools TODO\\n\"\n"
+ "\"glslang TODO\\n\"\n"
+ )
+endif()

function(define_pkg_config_file NAME LIBS)
add_custom_target(${NAME}-pkg-config ALL
diff --git a/glslc/CMakeLists.txt b/glslc/CMakeLists.txt
index 31664d1..bcd7cb6 100644
--- a/glslc/CMakeLists.txt
+++ b/glslc/CMakeLists.txt
@@ -53,7 +53,9 @@ shaderc_default_compile_options(glslc_exe)
target_include_directories(glslc_exe PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/.. ${spirv-tools_SOURCE_DIR}/include)
set_target_properties(glslc_exe PROPERTIES OUTPUT_NAME glslc)
target_link_libraries(glslc_exe PRIVATE glslc shaderc_util shaderc)
+if (SHADERC_ENABLE_THIRD_PARTY)
add_dependencies(glslc_exe build-version)
+endif()

shaderc_add_tests(
TEST_PREFIX glslc
diff --git a/libshaderc_util/src/compiler.cc b/libshaderc_util/src/compiler.cc
index 8a8d12b..92d8279 100644
--- a/libshaderc_util/src/compiler.cc
+++ b/libshaderc_util/src/compiler.cc
@@ -20,7 +20,7 @@
#include
#include

-#include "SPIRV/GlslangToSpv.h"
+#include "glslang/SPIRV/GlslangToSpv.h"
#include "libshaderc_util/format.h"
#include "libshaderc_util/io_shaderc.h"
#include "libshaderc_util/message.h"
```

Of course, this has several caveats as is:

- There is no detection of the presence of these dependencies, which means it breaks at compilation instead of configure if you haven't manually set `glslang_SOURCE_DIR` and `spirv-tools_SOURCE_DIR` to the appropriate include path, and likewise the link library is missing. Usually, you want to use `find_library()`, but given how messy the Khronos dependencies are deployed, this might be a challenge. Of course, there is also the hope that these dependencies end up deploying a `.pc` (pkg-config) containing version, link libraries, include paths, etc, which would solve all of this.
- The versions are not extracted (pkg-config would help here again) for `build-version.inc`, but they are only used for a help print, so I skipped them for now
- The glslang include breaks the third party code path, but this is the appropriate path if glslang is installed.

I don't know how we can proceed from here, but the current state really is terrible.