cmake: create own precompiled headers for tests

As soon as we split up our CMakeBuild.txt build instructions, we will be
unable to simply link against the git2 library's precompiled header from
other targets. To avoid this future breakage, create a new precompiled
header for our test suite. Next to being compatible with the split, this
enables us to also include additional files like the clar headers, which
may help speeding up compilation of the test suite.

CMakeLists.txt

@@ -693,6 +693,10 @@ IF (BUILD_CLAR)
 	FILE(GLOB_RECURSE SRC_TEST ${CLAR_PATH}/*/*.c ${CLAR_PATH}/*/*.h)
 	SET(SRC_CLAR "${CLAR_PATH}/main.c" "${CLAR_PATH}/clar_libgit2.c" "${CLAR_PATH}/clar_libgit2_trace.c" "${CLAR_PATH}/clar_libgit2_timer.c" "${CLAR_PATH}/clar.c")
 
+	IF(MSVC_IDE)
+		LIST(APPEND SRC_CLAR "${CLAR_PATH}/precompiled.c")
+	ENDIF()
+
 	ADD_CUSTOM_COMMAND(
 		OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/clar.suite
 		COMMAND ${PYTHON_EXECUTABLE} generate.py -o "${CMAKE_CURRENT_BINARY_DIR}" -f -xonline -xstress .
@@ -723,6 +727,7 @@ IF (BUILD_CLAR)
 	IF (MSVC_IDE)
 		# Precompiled headers
 		SET_TARGET_PROPERTIES(libgit2_clar PROPERTIES COMPILE_FLAGS "/Yuprecompiled.h /FIprecompiled.h")
+		SET_SOURCE_FILES_PROPERTIES("${CLAR_PATH}/tests/precompiled.c" COMPILE_FLAGS "/Ycprecompiled.h")
 	ENDIF ()
 
 	ENABLE_TESTING()

tests/precompiled.c

@@ -0,0 +1 @@
+#include "precompiled.h"

tests/precompiled.h

@@ -0,0 +1,4 @@
+#include "common.h"
+#include "git2.h"
+#include "clar.h"
+#include "clar_libgit2.h"