Fix Findlibdw.cmake and Findlibelf.cmake (#764)

- improve pkg-config and explicit search fallback

cmake/Modules/Findlibdw.cmake

@@ -25,8 +25,12 @@
 find_package(PkgConfig)
 
 if(PkgConfig_FOUND)
+    set(ENV{PKG_CONFIG_SYSTEM_INCLUDE_PATH} "")
     pkg_check_modules(DW libdw)
-    if(DW_FOUND)
+
+    if(DW_FOUND
+       AND DW_INCLUDE_DIRS
+       AND DW_LIBRARIES)
         set(libdw_INCLUDE_DIR
             "${DW_INCLUDE_DIRS}"
             CACHE FILEPATH "libdw include directory")
@@ -36,7 +40,7 @@ if(PkgConfig_FOUND)
     endif()
 endif()
 
-if(NOT PkgConfig_FOUND OR NOT DW_FOUND)
+if(NOT libdw_INCLUDE_DIR OR NOT libdw_LIBRARY)
     find_path(
         libdw_ROOT_DIR
         NAMES include/elfutils/libdw.h

cmake/Modules/Findlibelf.cmake

@@ -25,9 +25,12 @@
 find_package(PkgConfig)
 
 if(PkgConfig_FOUND)
+    set(ENV{PKG_CONFIG_SYSTEM_INCLUDE_PATH} "")
     pkg_check_modules(ELF libelf)
 
-    if(ELF_FOUND)
+    if(ELF_FOUND
+       AND ELF_INCLUDE_DIRS
+       AND ELF_LIBRARIES)
         set(libelf_INCLUDE_DIR
             "${ELF_INCLUDE_DIRS}"
             CACHE FILEPATH "libelf include directory")
@@ -37,18 +40,18 @@ if(PkgConfig_FOUND)
     endif()
 endif()
 
-if(NOT PkgConfig_FOUND OR NOT ELF_FOUND)
+if(NOT libelf_INCLUDE_DIR OR NOT libelf_LIBRARY)
     find_path(
         libelf_ROOT_DIR
-        NAMES include/gelf.h include/elf.h
+        NAMES include/elf.h
         HINTS ${libelf_ROOT}
         PATHS ${libelf_ROOT})
 
     mark_as_advanced(libelf_ROOT_DIR)
 
     find_path(
         libelf_INCLUDE_DIR
-        NAMES gelf.h elf.h
+        NAMES elf.h
         HINTS ${libelf_ROOT}
         PATHS ${libelf_ROOT}
         PATH_SUFFIXES include)