build: use parallelization except on Windows

This is similar to LizardByte/Sunshine#3361, except now multithreaded
processing is not used on Windows.

Unlike LizardByte/Sunshine#3361, we're now enabling parallelization
for both dot graph generation and other doxygen processing tasks as well
(except on Windows).

To check for Windows, we use the macro `CMAKE_HOST_WIN32` rather than
the more well-known `WIN32`, because what matters here is the host
platform, not the target platform. In most cases, these will be the
same, but the issue with parallelization is related to Windows as a
host, not a target. In other words, using Windows to build the
documentation for a macOS project should still be single-threaded.

CMakeLists.txt

@@ -92,13 +92,23 @@ add_custom_command(
 file(RELATIVE_PATH DOXYGEN_BUILD_DIR_RELATIVE "${SOURCE_DOCS_DIR}" "${DOXYGEN_BUILD_DIR_CMAKE}")
 message(STATUS "DOXYGEN_BUILD_DIR_RELATIVE: ${DOXYGEN_BUILD_DIR_RELATIVE}")
 
+if(CMAKE_HOST_WIN32)
+    # On Windows, we have to build the documentation using only a single thread to
+    # avoid the build mysteriously taking forever.
+    # See https://github.com/doxygen/doxygen/issues/9694
+    set(DOXYGEN_NUM_THREADS 1)
+else()
+    set(DOXYGEN_NUM_THREADS 0)
+endif()
+
 # build docs
 add_custom_target(docs ALL
         COMMENT "Building Doxygen documentation"
         WORKING_DIRECTORY "${SOURCE_DOCS_DIR}"
         COMMAND ${CMAKE_COMMAND} -E env
             READTHEDOCS_OUTPUT=${DOXYGEN_BUILD_DIR_RELATIVE}
             READTHEDOCS_VERSION=${DOXYGEN_PROJECT_VERSION}
+            DOXYCONFIG_THREADS=${DOXYGEN_NUM_THREADS}
             ${DOXYGEN_EXECUTABLE} doxyconfig-Doxyfile
         VERBATIM
         DEPENDS FONT_AWESOME_FILES

doxyconfig-Doxyfile

@@ -107,7 +107,7 @@ DISABLE_INDEX = NO
 DOCBOOK_OUTPUT = docbook
 DOT_GRAPH_MAX_NODES = 50
 DOT_IMAGE_FORMAT = svg
-DOT_NUM_THREADS = 1
+DOT_NUM_THREADS = $(DOXYCONFIG_THREADS)
 EXTRACT_ALL = NO
 FULL_SIDEBAR = NO
 GENERATE_HTML = YES
@@ -122,7 +122,7 @@ MACRO_EXPANSION = YES
 MAN_OUTPUT = man
 MARKDOWN_ID_STYLE = GITHUB
 MARKDOWN_SUPPORT = YES
-NUM_PROC_THREADS = 1
+NUM_PROC_THREADS = $(DOXYCONFIG_THREADS)
 PROJECT_NUMBER = $(READTHEDOCS_VERSION)
 OUTPUT_DIRECTORY = $(READTHEDOCS_OUTPUT)
 RECURSIVE = YES