CMakeLists: fix build with old compilers

The libubootenv code uses some C99 constructs, but old compilers such
as gcc 4.7 don't default to -std=c99. This commit makes sure libubootenv
is built with -std=gnu99.

Fixes:

src/uboot_env.c:897:5: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
     for (int i = 0; i < strlen(pval); i++) {
     ^

Signed-off-by: Pierre-Jean Texier <pjtexier@koncepto.io>

CMakeLists.txt

@@ -8,6 +8,8 @@ set(VERSION	"0.1")
 SET(SOVERSION "${VERSION}")
 add_definitions(-DVERSION="${VERSION}")
 
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99")
+
 #set(CMAKE_C_FLAGS_DEBUG "-g")
 include_directories ("${PROJECT_SOURCE_DIR}/src")
 add_subdirectory (src)