[fftw3] use proper CMake target and move builtin-build to subdir

Author: ferdymercury

.github/workflows/root-ci-config/buildconfig/alma10-clang_ninja.txt

@@ -1,6 +1,7 @@
 CMAKE_C_COMPILER=clang
 CMAKE_CXX_COMPILER=clang++
 CMAKE_GENERATOR=Ninja
+builtin_fftw3=ON
 builtin_freetype=ON
 builtin_ftgl=ON
 builtin_gif=ON

builtins/fftw3/CMakeLists.txt

@@ -0,0 +1,30 @@
+set(ROOT_FFTW_VERSION 3.3.10)
+message(STATUS "Downloading and building FFTW version ${ROOT_FFTW_VERSION}")
+set(ROOT_FFTW_PREFIX ${CMAKE_BINARY_DIR}/builtins/FFTW3-prefix)
+set(ROOT_FFTW_LIBRARY ${ROOT_FFTW_PREFIX}/lib/libfftw3.a)
+ExternalProject_Add(
+    BUILTIN_FFTW3
+    PREFIX ${ROOT_FFTW_PREFIX}
+    URL ${lcgpackages}/fftw-${ROOT_FFTW_VERSION}.tar.gz
+    URL_HASH SHA256=56c932549852cddcfafdab3820b0200c7742675be92179e59e6215b340e26467
+    CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR>
+    BUILD_COMMAND make CFLAGS=-fPIC
+    LOG_DOWNLOAD 1 LOG_CONFIGURE 1 LOG_BUILD 1 LOG_INSTALL 1 LOG_OUTPUT_ON_FAILURE 1
+    BUILD_IN_SOURCE 1
+    BUILD_BYPRODUCTS ${ROOT_FFTW_LIBRARY}
+    TIMEOUT 600
+)
+
+set(ROOT_FFTW_INCLUDE_DIR ${ROOT_FFTW_PREFIX}/include)
+
+file(MAKE_DIRECTORY ${ROOT_FFTW_INCLUDE_DIR})
+add_library(FFTW::Double IMPORTED STATIC GLOBAL)
+add_dependencies(FFTW::Double BUILTIN_FFTW3)
+set_target_properties(FFTW::Double PROPERTIES
+    IMPORTED_LOCATION ${ROOT_FFTW_LIBRARY}
+    INTERFACE_INCLUDE_DIRECTORIES ${ROOT_FFTW_INCLUDE_DIR})
+
+set(FFTW_INCLUDE_DIRS ${ROOT_FFTW_INCLUDE_DIR} PARENT_SCOPE)
+set(FFTW_FOUND ON PARENT_SCOPE)
+set(FFTW_VERSION ${ROOT_FFTW_VERSION} PARENT_SCOPE)
+set(FFTW_LIBRARIES ${ROOT_FFTW_LIBRARY} PARENT_SCOPE)