CMake: Allow version to be explicitly set without requiring git (#384)

The main CMakeLists.txt was updated to allow explicitly setting the
version for the project by setting the `ADS_VERSION` variable (e.g.
cmake .. -DADS_VERSION=1.0.0).

The default behavior is to determine the version by reading the
information from git. Adding the option to override this variable at
configure time allows the library to be built outside of its git
repository, such as when the code is vendored directly into another
project and added using `add_subdirectory`.

Author: rminderhoud

CMakeLists.txt

@@ -1,20 +1,41 @@
 cmake_minimum_required(VERSION 3.5)
-set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules ${CMAKE_MODULE_PATH})
-include(GetGitRevisionDescription)
-git_describe(GitTagVersion --tags)
-string(REGEX REPLACE "^([0-9]+)\\..*" "\\1" VERSION_MAJOR "${GitTagVersion}")
-string(REGEX REPLACE "^[0-9]+\\.([0-9]+).*" "\\1" VERSION_MINOR "${GitTagVersion}")
-string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" VERSION_PATCH "${GitTagVersion}")
-set(VERSION_SHORT "${VERSION_MAJOR}.${VERSION_MINOR}.${VERSION_PATCH}")
+
+# By default, the version information is extracted from the git index. However,
+# we can override this behavior by explicitly setting ADS_VERSION and
+# skipping the git checks. This is useful for cases where this project is being
+# used independently of its original git repo (e.g. vendored in another project)
+if(NOT ADS_VERSION)
+    set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules ${CMAKE_MODULE_PATH})
+    include(GetGitRevisionDescription)
+    git_describe(GitTagVersion --tags)
+    string(REGEX REPLACE "^([0-9]+)\\..*" "\\1" VERSION_MAJOR "${GitTagVersion}")
+    string(REGEX REPLACE "^[0-9]+\\.([0-9]+).*" "\\1" VERSION_MINOR "${GitTagVersion}")
+    string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" VERSION_PATCH "${GitTagVersion}")
+    set(VERSION_SHORT "${VERSION_MAJOR}.${VERSION_MINOR}.${VERSION_PATCH}")
+else()
+    string(REGEX MATCHALL "[\.]" VERSION_DOT_MATCHES ${ADS_VERSION})
+    list(LENGTH VERSION_DOT_MATCHES VERSION_DOT_COUNT)
+    if(VERSION_DOT_COUNT EQUAL 2)
+        set(VERSION_SHORT ${ADS_VERSION})
+    else()
+        message(FATAL_ERROR "ADS_VERSION must be in major.minor.patch format, e.g. 3.8.1. Got ${ADS_VERSION}")
+    endif()
+endif()
+
+
 project(QtADS LANGUAGES CXX VERSION ${VERSION_SHORT})
+
 option(BUILD_STATIC "Build the static library" OFF)
 option(BUILD_EXAMPLES "Build the examples" ON)
+
 if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "4")
     set(ads_PlatformDir "x86")
 else()
     set(ads_PlatformDir "x64")
 endif()
+
 add_subdirectory(src)
+
 if(BUILD_EXAMPLES)
     add_subdirectory(examples)
     add_subdirectory(demo)