populate slang-tag-version with cmake (#4611)

At the moment it is always "unknown"

slang.h

@@ -1658,15 +1658,18 @@ extern "C"
         void*       userData);
 
     /*!
-    @brief Get the build version 'tag' string. The string is the same as produced via `git describe --tags`
-    for the project. If Slang is built separately from the automated build scripts
-    the contents will by default be 'unknown'. Any string can be set by changing the
-    contents of 'slang-tag-version.h' file and recompiling the project.
-
-    This function will return exactly the same result as the method getBuildTag string on IGlobalSession.
-
-    An advantage of using this function over the method is that doing so does not require the creation of
-    a session, which can be a fairly costly operation.
+    @brief Get the build version 'tag' string. The string is the same as
+    produced via `git describe --tags --match v*` for the project. If such a
+    version could not be determined at build time then the contents will be
+    0.0.0-unknown. Any string can be set by passing
+    -DSLANG_VERSION_FULL=whatever during the cmake invocation.
+
+    This function will return exactly the same result as the method
+    getBuildTagString on IGlobalSession.
+
+    An advantage of using this function over the method is that doing so does
+    not require the creation of a session, which can be a fairly costly
+    operation.
 
     @return The build tag string
     */

source/slang/CMakeLists.txt

@@ -249,7 +249,7 @@ target_include_directories(
 #
 # Slang itself
 #
-configure_file(${slang_SOURCE_DIR}/slang-tag-version.h.in slang-tag-version.h)
+configure_file(${slang_SOURCE_DIR}/slang-tag-version.h.in slang-version-header/slang-tag-version.h)
 slang_add_target(
     .
     ${SLANG_LIB_TYPE}
@@ -267,10 +267,10 @@ slang_add_target(
     # slang.h is in the project root, so include that directory in the interface
     # for slang
     INCLUDE_DIRECTORIES_PUBLIC ${slang_SOURCE_DIR}
-    INCLUDE_DIRECTORIES_PRIVATE ${CMAKE_CURRENT_BINARY_DIR}
+    INCLUDE_DIRECTORIES_PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/slang-version-header
     EXPORT_MACRO_PREFIX SLANG
     INSTALL
-    PUBLIC_HEADERS ${slang_SOURCE_DIR}/slang*.h
+    PUBLIC_HEADERS ${slang_SOURCE_DIR}/slang*.h ${CMAKE_CURRENT_BINARY_DIR}/slang-version-header/*.h
 )
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")

source/slang/slang.cpp

@@ -47,7 +47,7 @@
 
 #include "slang-check-impl.h"
 
-#include "../../slang-tag-version.h"
+#include "slang-tag-version.h"
 
 #include <sys/stat.h>
 
@@ -124,7 +124,7 @@ namespace Slang {
 
 const char* getBuildTagString()
 {
-    if (UnownedStringSlice(SLANG_TAG_VERSION) == "unknown")
+    if (UnownedStringSlice(SLANG_TAG_VERSION) == "0.0.0-unknown")
     {
         // If the tag is unknown, then we will try to get the timestamp of the shared library
         // and use that as the version string, so that we can at least return something