CMake-only computation of whether librt is needed. (#374)

CMakeLists.txt

@@ -16,12 +16,13 @@ MESSAGE(STATUS "CMake v${CMAKE_VERSION}")
 
 IF (CMAKE_SYSTEM_NAME STREQUAL "Linux")
     # If using older glibc, need to link with -lrt.  See clock_getres(2).
-    EXECUTE_PROCESS(
-        COMMAND ${PROJECT_SOURCE_DIR}/print-glibc-version.sh ${CMAKE_C_COMPILER}
-        OUTPUT_VARIABLE GLIBC_VERSION)
-    IF(NOT GLIBC_VERSION EQUAL "" AND GLIBC_VERSION VERSION_LESS 2.17)
-        SET(LIBS ${LIBS} rt)
-    ENDIF()
+    include(CheckSymbolExists)
+    check_symbol_exists(clock_getres "time.h" HAS_clock_getres_WITHOUT_LIBRT)
+
+    if(NOT HAS_clock_getres_WITHOUT_LIBRT)
+        find_library(RT_LIBRARY rt)
+        set(NEED_LIBRT_FOR_clock_getres ON)
+    endif()
 ELSEIF (CMAKE_SYSTEM_NAME STREQUAL "Android")
     # for android-ndk >= r19b
     set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY "BOTH")

print-glibc-version.sh

@@ -1,13 +0,0 @@
-#!/bin/sh
-#
-# Determine glibc version and print it to stdout.  I have to resort to
-# using a shell script because it is taking too long to figure out how
-# to do this properly in cmake.
-
-CC=$1
-if [ "" = "$CC" ]; then
-    CC=gcc
-fi
-
-$CC -print-file-name=libc.so.6 \
-    | perl -plne '$_ = readlink if -H; s/\.so$// && s/.*-//'

src/liblsquic/CMakeLists.txt

@@ -119,6 +119,10 @@ SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DXXH_HEADER_NAME=\\\"lsquic_xxhash.h\\\"")
 SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DLSQPACK_ENC_LOGGER_HEADER=\\\"lsquic_qpack_enc_logger.h\\\"")
 SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DLSQPACK_DEC_LOGGER_HEADER=\\\"lsquic_qpack_dec_logger.h\\\"")
 
+if(NEED_LIBRT_FOR_clock_getres)
+    target_link_libraries(lsquic PRIVATE ${RT_LIBRARY})
+endif()
+
 IF(LSQUIC_SHARED_LIB)
   add_library(lsquic SHARED ${lsquic_STAT_SRCS})
   TARGET_LINK_LIBRARIES(lsquic PRIVATE ${BORINGSSL_LIB_ssl} ${BORINGSSL_LIB_crypto} ${ZLIB_LIB})