Raise C++ version to C++14

GoogleTest requires C++14 since 1.13.0 (see release notes [1]).

This is required for running CI on macOS with GitHub Actions in a
later commit because Homebrew will install GoogleTest 1.15.2 [2].

[1] https://github.com/google/googletest/releases/tag/v1.13.0
[2] https://formulae.brew.sh/formula/googletest

Makefile.inc

@@ -2,7 +2,7 @@
 CXX ?= g++
 CXXWARNINGS ?= -Wall -Wextra -Wundef -pedantic
 CXXINTRINSICS ?= -march=native
-CXXFEATURES ?= -funsafe-math-optimizations -fno-math-errno -std=c++11
+CXXFEATURES ?= -funsafe-math-optimizations -fno-math-errno -std=c++14
 CXXFLAGS ?= ${CXXWARNINGS} ${CXXINTRINSICS} ${CXXFEATURES} -g -O3
 
 # Delete this if OpenMP is not available (e.g., OS X without gcc)

tests/Makefile

@@ -8,7 +8,7 @@ GTEST_LDFLAGS = `pkg-config --libs gtest_main`
 SOURCES = $(wildcard math/gtest_*.cc) $(wildcard mve/gtest_*.cc) $(wildcard sfm/gtest_*.cc) $(wildcard util/gtest_*.cc) $(wildcard fssr/gtest_*.cc)
 INCLUDES = -I${MVE_ROOT}/libs ${GTEST_CFLAGS}
 CXXWARNINGS = -Wall -Wextra -pedantic -Wno-sign-compare
-CXXFLAGS = -std=c++11 -pthread ${CXXWARNINGS} ${INCLUDES}
+CXXFLAGS = -std=c++14 -pthread ${CXXWARNINGS} ${INCLUDES}
 LDLIBS += ${GTEST_LDFLAGS} ${LIBJPEG_LDFLAGS} ${LIBPNG_LDFLAGS} ${LIBTIFF_LDFLAGS}
 
 test: ${SOURCES:.cc=.o} libmve_fssr.a libmve_sfm.a libmve.a libmve_util.a