Explicitly specify the project is a C project.

CMake assumes the default project uses both C and C++, and therefore
will fail if both a C and a C++ compiler isn't found. This essentially
blocks pure C projects from using msgpack-c if they also don't have a
C++ compiler.

Just specifying that the project is "C" isn't possible though, as the
test themselves use C++. The workaround here is that we specify the
project needs both a C and C++ compiler if tests are used, but only a C
compiler if the tests aren't used.

Author: dundargoc

CMakeLists.txt

@@ -5,7 +5,14 @@ IF ((CMAKE_VERSION VERSION_GREATER 3.1) OR
     CMAKE_POLICY(SET CMP0054 NEW)
 ENDIF ()
 
-PROJECT (msgpack-c)
+OPTION (MSGPACK_BUILD_TESTS "Build msgpack tests." ON)
+OPTION (MSGPACK_GEN_COVERAGE "Enable running gcov to get a test coverage report." OFF)
+
+if(MSGPACK_BUILD_TESTS)
+    PROJECT (msgpack-c)
+else()
+    PROJECT (msgpack-c C)
+endif()
 
 FILE (READ ${CMAKE_CURRENT_SOURCE_DIR}/include/msgpack/version_master.h contents)
 STRING (REGEX MATCH "#define MSGPACK_VERSION_MAJOR *([0-9a-zA-Z_]*)" NULL_OUT ${contents})
@@ -74,14 +81,6 @@ IF (MSGPACK_CHAR_SIGN)
    SET (CMAKE_C_FLAGS "-f${MSGPACK_CHAR_SIGN}-char ${CMAKE_C_FLAGS}")
 ENDIF ()
 
-FIND_PACKAGE (GTest)
-FIND_PACKAGE (ZLIB)
-FIND_PACKAGE (Threads)
-IF (GTEST_FOUND AND ZLIB_FOUND AND THREADS_FOUND)
-    OPTION (MSGPACK_BUILD_TESTS "Build msgpack tests." ON)
-    OPTION (MSGPACK_GEN_COVERAGE "Enable running gcov to get a test coverage report." OFF)
-ENDIF ()
-
 IF (DEFINED BUILD_SHARED_LIBS)
     IF (BUILD_SHARED_LIBS)
         IF (DEFINED MSGPACK_ENABLE_SHARED AND NOT MSGPACK_ENABLE_SHARED)