package/gmp: fix target build with host gcc 4.9

GMP does not build if the host gcc is v4.9 due to the following error

  gen-sieve.c: In function 'setmask':
  gen-sieve.c:99:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
     for (unsigned i = 0; i < 2 * a * b; ++i)
     ^
  gen-sieve.c:99:3: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code

The gen-sieve utility was added in GMP v6.3.0. It is built using
CC_FOR_BUILD (host compiler) during cross compilation as it generates
build files. Autoconf does not have a macro for add -std=c99 to
CC_FOR_BUILD, so it must be set manually. For the target, it is set
correctly thanks to the AC_PROG_CC_C99 macro.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit 9553dc9a55c13916c26aaae4fac037d35ddacca3)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gmp/gmp.mk

@@ -14,6 +14,8 @@ GMP_CPE_ID_VENDOR = gmplib
 GMP_DEPENDENCIES = host-m4
 HOST_GMP_DEPENDENCIES = host-m4
 
+GMP_CONF_ENV += CC_FOR_BUILD="$(HOSTCC) -std=c99"
+
 # GMP doesn't support assembly for coldfire or mips r6 ISA yet
 # Disable for ARM v7m since it has different asm constraints
 ifeq ($(BR2_m68k_cf)$(BR2_MIPS_CPU_MIPS32R6)$(BR2_MIPS_CPU_MIPS64R6)$(BR2_ARM_CPU_ARMV7M),y)