Respect CFLAGS and CXXFLAGS provided by the package manager.
https://bugs.gentoo.org/952032

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9487ab98a594..a8e1bed7a527 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -68,14 +68,6 @@ TARGET_LINK_LIBRARIES(loudgain
   ${LTAG_LIBRARIES}
 )
 
-SET_TARGET_PROPERTIES(loudgain PROPERTIES
-  COMPILE_FLAGS "-Wall -pedantic -g"
-)
-
-SET(CMAKE_C_FLAGS "-std=gnu99 -D_GNU_SOURCE")
-
-SET(CMAKE_CXX_FLAGS "-std=gnu++11 -D_GNU_SOURCE")
-
 INSTALL(TARGETS loudgain DESTINATION ${CMAKE_INSTALL_PREFIX}/bin)
 
 INSTALL(FILES