Probe for C99 standard available

Similar to the -std=c++0x flag for C++ code, we probe for, and add if
available, -std=c99 for C code, since the compiler may generate
different initialization code for this variant.

The reporting from the bundled CMake modules is not as polished as for
probing for C++ flags, but not bad enough to warrant rewriting the code
from scratch just to fix the output.

CMakeLists.txt

@@ -4,6 +4,7 @@ cmake_minimum_required (VERSION 2.8)
 project (opm-core)
 set (opm-core_VERSION_MAJOR 0)
 set (opm-core_VERSION_MINOR 3)
+enable_language (C)
 enable_language (CXX)
 
 # build debug by default
@@ -28,6 +29,8 @@ include (OpmFind)
 
 # dependencies
 list (APPEND opm-core_DEPS
+	# compile with C99 support if available
+	"C99"
 	# compile with C++0x/11 support if available
 	"CXX11Features"
 	# matrix library

cmake/Modules/FindC99.cmake

@@ -0,0 +1,25 @@
+# - Module that checks for supported C99 features.
+
+# macro to only add option once
+include (AddOptions)
+
+# try to use compiler flag -std=c99
+set (C_STD99_FLAGS "-std=c99")
+
+# incidently, the C++ test is so simple that it can be used to compile C as well
+include (CheckCCompilerFlag)
+check_c_compiler_flag (${C_STD99_FLAGS} HAVE_C99)
+
+# add option if we are capable
+if (HAVE_C99)
+  add_options (C ALL_BUILDS "${C_STD99_FLAGS}")
+else (HAVE_C99)
+  set (C_STD99_FLAGS)
+endif (HAVE_C99)
+
+# handle quiet and required
+include (FindPackageHandleStandardArgs)
+find_package_handle_standard_args (C99
+  DEFAULT_MSG
+  C_STD99_FLAGS
+  )

opm-core-config.cmake.in

@@ -42,4 +42,9 @@ string (REPLACE "@CXX_STD0X_FLAGS@" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 string (STRIP "${CMAKE_CXX_FLAGS}" CMAKE_CXX_FLAGS)
 set (CMAKE_CXX_FLAGS "@CXX_STD0X_FLAGS@ ${CMAKE_CXX_FLAGS}")
 
+# same as above, but for C99
+string (REPLACE "@C_STD99_FLAGS@" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+string (STRIP "${CMAKE_C_FLAGS}" CMAKE_C_FLAGS)
+set (CMAKE_C_FLAG "@C_STD99_FLAGS@ ${CMAKE_C_FLAGS}")
+
 # this is the contents of config.h as far as our probes can tell: