Add and use FindLibClang module for c-picker setup

c-picker uses libclang to parse the code. If compatible libclang is not
available in the system's default library directory c-picker needs to
have CLANG_LIBRARY_PATH environment variable pointing to the suitable
directory. The new module tries to find this directory and the
environment variable is now set during the c-picker call.

Change-Id: I09327b2d09620884ee72ff831ead546a0d10b900
Signed-off-by: Imre Kis <imre.kis@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 86029e0..bcfb234 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -23,12 +23,40 @@
 set(CPICKER_CACHE_PATH ${CMAKE_CURRENT_BINARY_DIR}/cpicker_cache CACHE PATH "Directory of c-picker generated file")
 set(UNIT_TEST_COMMON_SOURCES ${CMAKE_CURRENT_LIST_DIR}/common/main.cpp)
 set(CMAKE_CXX_STANDARD 11)
+set(CLANG_LIBRARY_PATH_HELP "libclang directory for c-picker")
 
 # Checking TF-A
 if (NOT TF_A_PATH)
 	message(FATAL_ERROR "TF_A_PATH is not set")
 endif()
 
+# Trying to set CLANG_LIBRARY_PATH using the following methods
+# 1. Using cache or command line definition
+#    Show warning if environment variable is also set but has different value
+# 2. Copying the value of CLANG_LIBRARY_PATH environment variable if set
+# 3. find_package (llvm-config, common paths or Windows registry)
+# If none of the above steps succeeded CMake emits a fatal error and stops
+if (DEFINED CLANG_LIBRARY_PATH)
+	message(STATUS "Using CLANG_LIBRARY_PATH from CMake variable (command line or cache)")
+
+	if (DEFINED ENV{CLANG_LIBRARY_PATH})
+		if (NOT (${CLANG_LIBRARY_PATH} STREQUAL $ENV{CLANG_LIBRARY_PATH}))
+			message(WARNING "Both CLANG_LIBRARY_PATH CMake and environment variables are set but have different values")
+		endif()
+	endif()
+else()
+	if (DEFINED ENV{CLANG_LIBRARY_PATH})
+		message(STATUS "Setting CLANG_LIBRARY_PATH based on environment variable")
+		set(CLANG_LIBRARY_PATH $ENV{CLANG_LIBRARY_PATH} CACHE PATH ${CLANG_LIBRARY_PATH_HELP})
+	else()
+		message(STATUS "Setting CLANG_LIBRARY_PATH based on find_package")
+		find_package(LibClang REQUIRED)
+		set(CLANG_LIBRARY_PATH ${LibClang_LIBRARY_DIRS} CACHE PATH ${CLANG_LIBRARY_PATH_HELP})
+	endif()
+endif()
+
+message(STATUS "CLANG_LIBRARY_PATH has been set to ${CLANG_LIBRARY_PATH}")
+
 # Checking c-picker
 find_program(CPICKER_COMMAND "c-picker")
 if (NOT CPICKER_COMMAND)