Specify CXX as project language for CMake >= 3.4 (#1027)

CMakeLists.txt

@@ -12,7 +12,12 @@ if (POLICY CMP0048)
   cmake_policy(SET CMP0048 NEW)
 endif()
 
-project(pybind11)
+# CMake versions < 3.4.0 do not support try_compile/pthread checks without C as active language.
+if(CMAKE_VERSION VERSION_LESS 3.4.0)
+  project(pybind11)
+else()
+  project(pybind11 CXX)
+endif()
 
 # Check if pybind11 is being used directly or via add_subdirectory
 set(PYBIND11_MASTER_PROJECT OFF)

tests/CMakeLists.txt

@@ -12,7 +12,7 @@ option(PYBIND11_WERROR  "Report all warnings as errors"  OFF)
 if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
     # We're being loaded directly, i.e. not via add_subdirectory, so make this
     # work as its own project and load the pybind11Config to get the tools we need
-    project(pybind11_tests)
+    project(pybind11_tests CXX)
 
     find_package(pybind11 REQUIRED CONFIG)
 endif()