complibs/ppl: fix build when local system has libgmp already installed

Basically, the ppl configure script wasn't properly setting
CFLAGS/CXXFLAGS/LDFLAGS to point at the ctng-built gmp, so ppl tried to
use the system gmp headers instead.

This patch fixes that for all PPL versions supported by crosstools-ng.

Signed-off-by: Solomon Peachy <pizza@shaftnet.org>
[yann.morin.1998@free.fr: slightly touch the commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <20130715230656.GB23382@shaftnet.org>
Patchwork-Id: 259300

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_LE=y

 CT_ARCH_FLOAT_SW=y

 CT_ARCH_mips=y

 CT_TARGET_VENDOR="sde"

 CT_BINUTILS_V_2_21_1a=y

 CT_CC_V_4_4_6=y

 # CT_CC_GCC_ENABLE_TARGET_OPTSPACE is not set

 CT_LIBC_none=y

 CT_DEBUG_gdb=y

 CT_GDB_V_6_8a=y

 CT_GMP_V_4_3_2=y

 CT_MPFR_V_2_4_2=y

 CT_PPL_V_0_10_2=y

 CT_CLOOG_V_0_15_10=y